iXX-*'V</V_^>-*V^'VX^ 


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UC-NRLF 


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THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

PRESENTED  BY 

PROF. CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


THE 

POWEE  OF   MOVEMENT 


IN 


PLANTS. 


THE 


POWER  OF  MOVEMENT 

PLANTS. 

BY  CHAELES  DAEWIN,  LL.D.,  F.E.S. 

ASSISTED  BY 

EEANCIS    DAEWIN. 


WITH    ILLUSTRATIONS. 


NEW  YOEK : 
D.    APPLETOIST    AND    COMPANY, 

1,    3,    AND   5    BOND    STREET. 

1881. 


0 


•QKll 


CONTENTS. 


INTRODUCTION Page  1-9 

CHAPTER    I. 

THE   ClRCUMNUTATING   MOVEMENTS   OF    SEEDLING    PLANTS. 

Brassica  oleracea,  circumnutation  of  the  radicle,  of  the  arched  hypo- 
cotyl  whilst  still  buried  beneath  the  ground,  whilst  rising  above 
the  ground  and  straightening  itself,  and  when  erect — Circumnu- 
tation of  the  cotyledons — Rate  of  movement — Analogous  obser- 
vations on  various  organs  in  species  of  Githago,  Gossypium, 
Oxalis,  Tropseolum,  Citrus,  ^Esculus,  of  several  Leguminous  and 
Cucurbitaceous  genera,  Opuntia,  Helianthiis,  Primula,  Cyclamen, 
Stapelia,  Cerinthe,  Nolana,  Solanum,  Beta,  Ricinus,  Quercus, 
Corylus,  Pinus,  Cycas,  Canna,  Allium,  Asparagus,  Phalaris,  Zea, 
A  vena,  Nephrodium,  and  Selaginella  10-66 

CHAPTER   II. 

GENERAL  CONSIDERATIONS  ON  THE  MOVEMENTS  AND  GROWTH  OF 
SEEDLING  PLANTS. 

Generality  of  the  circumnutating  movement — Radicles,  their  cir- 
cumnutation of  service — Manner  in  which  they  penetrate  the 
ground — Manner  in  which  hypocotyls  and  other  organs  break 
through  the  ground  by  being  arched — Singular  manner  of  ger- 
mination in  Megarrhiza,  &c. — Abortion  of  cotyledons — Circum- 
nutation  of  hypocotyls  and  epicotyls  whilst  still  buried  and 
arched — Their  power  of  straightening  themselves — Bursting  of 
the  seed-coats — inherited  effect  of  the  arching  process  in  hypo- 


VI  CONTENTS. 

gean  hypocotyls — Circumnutation  of  hypocotyls  and  epicotyls 
when  erect — Circumnutation  of  cotyledons — Pulvini  or  joints  of 
cotyledons,  duration  of  their  activity,  rudimentary  in  Oxalis 
corniculata,  their  development — Sensitiveness  of  cotyledons  to 
light  and  consequent  disturbance  of  their  periodic  movements — 
Sensitiveness  of  cotyledons  to  contact Page  07-128 


CHAPTEE    III. 

SENSITIVENESS  OF  THE  APEX  OF  THE  RADICLE  TO  CONTACT  AND 
TO  OTHER  IRRITANTS. 

Manner  in  which  radicles  bend  when  they  encounter  an  obstacle  in 
the  soil — Vicia  faba,  tips  of  radicles  highly  sensitive  to  con- 
tact and  other  irritants — Effects  of  too  high  a  temperature — 
Power  of  discriminating  between  objects  attached  on  opposite 
sides  —  Tips  of  secondary  radicles  sensitive  —  Fisum,  tips  of 
radicles  sensitive — Effects  of  such  sensitiveness  in  overcoming 
geotropism — Secondary  radicles  —  Phaseolus,  tips  of  radicles 
hardly  sensitive  to  contact,  but  highly  sensitive  to  caustic  and 
to  the  removal  of  a  slice — Tropseolum — Gossypiura — Cucurbita 
— Eaphanus — JBecuhts,  tip  not  sensitive  to  slight  contact,  highly 
sensitive  to  caustic — Quercus,  tip  highly  sensitive  to  contact — 
Power  of  discrimination — Zea,  tip  highly  sensitive,  secondary 
radicles — Sensitiveness  of  radicles  to  moist  air — Summary  of 
chapter 129-200 


CHAPTER   IV. 

TlIE  CiRCUMNCTATING  MOVEMENTS  OF  THE  SEVERAL  PARTS  OF 
MATURE  PLANTS. 

Circumnutation  of  stems :  concluding  remarks  on — Circumnutation 
of  stolons :  aid  thus  afforded  in  winding  amongst  the  stems  of 
surrounding  plants — Circumnutation  of  flower-stems — Circum- 
nutation of  Dicotyledonous  leaves — Singular  oscillatory  move- 
ment of  leaves  of  Dionaea — Leaves  of  Cannabis  sink  at  night — 
Leaves  of  Gymnosperms — Of  Monocotyledons — Cryptogams — 
Concluding  remarks  on  the  Circumnutation  of  leaves  :  generally 
rise  in  the  evening  and  sink  in  the  morning  ..  ..  201-262 


CONTENTS.  Vii 

CHAPTER  V. 

MODIFIED  CIRCUMNUTATION  :   CLIMBING  PLANTS  ;  EPINASTIC  AND 
HYPONASTIC  MOVEMENTS. 

Circumautation  modified  through  innate  causes  or  through  the  action 
of  external  conditions — Innate  causes — Climbing  plants ;  simi- 
larity of  their  movements  with  those  of  ordinary  plants ;  in- 
creased amplitude;  occasional  points  of  difference — Epinastic 
growth  of  young  leaves — Hyponastic  growth  of  the  hypocotyls 
and  epicotyls  of  seedlings — Hooked  tips  of  climbing  and  other 
plants  due  to  modified  circumnutation — Ampelopsis  tricuspidata 
— Smithia  Pfundii — Straightening  of  the  tip  due  to  hyponasty — 
Epinastic  growth  and  circumnutation  of  the  flower-peduncles  of 
Trifolium  repens  and  Oxalis  carnosa Page  263-279 

CHAPTER   VI. 

MODIFIED  CIRCUMNUTATION  :  SLEEP  OR  NYCTITROPIC  MOVEMENTS, 
THEIR  USE:  SLEEP  OP  COTYLEDONS. 

Preliminary  sketch  of  the  sleep  or  nyctitropic  movements  of  leaves 
— Presence  of  pulvini — The  lessening  of  radiation  the  final  cause 
of  nictritropic  movements — Manner  of  trying  experiments  on 
leaves  of  Oxalis,  Arachis,  Cassia,  Melilotus,  Lotus  and  Marsilea, 
and  on  the  cotyledons  of  Mimosa — Concluding  remarks  on  radia- 
tion from  leaves — Small  differences  in  the  conditions  make  a 
great  difference  in  the  result — Description  of  the  nyctitropic 
position  and  movements  of  the  cotyledons  of  various  plants — 
List  of  species — Concluding  remarks — Independence  of  the 
nyctitropic  movements  of  the  leaves  and  cotyledons  of  the  same 
species — Reasons  for  believing  that  the  movements  have  been 
acquired  for  a  special  purpose  280-316 

CHAPTER   VII. 

MODIFIED  CIRCUMNUTATION:  NYCTITROPIC  OR  SLEEP  MOVEMENTS 
OF  LEAVES. 

Conditions  necessary  for  these  movements — List  of  Genera  and 
Families,  which  include  sleeping  plants — Description  of  the 
movements  in  the  several  Genera — Oxalis:  leaflets  folded  at 


Vlll  CONTENTS. 

night — Averrhoa :  rapid  movements  of  the  leaflets — Porlierja  : 
leaflets  close  when  plant  kept  very  dry — Tropaeolum  :  leaves  do 
not  sleep  unless  well  illuminated  dining  day — Lupinus :  various 
modes  of  sleeping — Melilotus :  singular  movements  of  terminal 
leaflet — Trifolium — Desmodium:  rudimentary  lateral  leaflets, 
movements  of,  not  developed  on  young  plants,  state  of  their 
pulvini — Cassia  :  complex  movements  of  the  leaflets — Bauhinia: 
leaves  folded  at  ni^ht — Mimosa  pudica :  compounded  move- 
ments of  leaves,  effect  of  darkness — Mimosa  albida,  reduced 
leaflets  of — Schrankia:  downward  movement  of  the  pinnaj — 
Marsilea:  the  only  cryptogam  known  to  sleep — Concluding 
remarks  and  summary — Nyctitropism  consists  of  modified  cir- 
cumnutation,  regulated  by  the  alternations  of  light  and  darkness 
— Shape  of  first  true  leaves  Page  317-417 


CHAPTER   VIII. 

MODIFIED  CIRCUMNUTATION  :  MOVEMENTS  EXCITED  BY  LIGHT. 

Distinction  between  heliotropism  nnd  the  effects  of  light  on  the 
periodicity  of  the  movements  of  leaves — Heliotropic  movements 
of  Beta,  Solanum,  Zea,  and  Avena — Heliotropic  movements 
towards  an  obscure  light  in  Apios,  Brassica,  Phalaris,  Tropzeo- 
lum,  and  Cassia — Apheliotropic  movements  of  tendrils  of  Big- 
nonia — Of  flower-peduncles  of  Cyclamen — Burying  of  the  pods 
— Heliotropism  and  apheliotropism  modified  forms  of  circumnu- 
tation — Steps  by  which  one  movement  is  converted  into  the 
other — Transversal-heliotropismus  or  diaheliotropism  influenced 
by  epinasty,  the  weight  of  the  part  and  apogeotropism — Apogeo- 
tropism  overcome  during  the  middle  of  the  day  by  diaheliotro- 
pism— Effects  of  the  weight  of  the  blades  of  cotyledons — So- 
called  diurnal  sleep — Chlorophyll  injured  by  intense  light — 
Movements  to  avoid  intense  light 418-448 

CHAPTER    IX. 

SENSITIVENESS  OF  PLANTS  TO  LIGHT  :  ITS  TRANSMITTED  EFFECTS. 

Uses  of  heliotropism — Insectivorous  and  climbing  plants  not  helio- 
tropic — Same  organ  heliotropic  at  one  age  and  not  at  another — 
Extraordinary  sensitiveness  of  some  plants  to  light — The  eflecta 


CONTENTS.  ix 

of  light  do  not  correspond  with  its  intensity — Effects  of  previous 
illumination — Time  required  for  the  action  of  light — After-effects 
of  light — Apogeotropism  acts  as  soon  as  light  fails — Accuracy 
with  which  plants  bend  to  the  light — This  dependent  on  the 
illumination  of  one  whole  side  of  the  part — Localised  sensitive- 
ness to  light  and  its  transmitted  effects — Cotyledons  of  Phalaris, 
manner  of  bending — Results  of  the  exclusion  of  light  from  their 
tips — Effects  transmitted  beneath  the  surface  of  the  ground — 
Lateral  illumination  of  the  tip  determines  the  direction  of  the 
curvature  of  the  base — Cotyledons  of  A  vena,  curvature  of  basal 
part  due  to  the  illumination  of  upper  part — Similar  results  with 
the  hypocotyls  of  Brassica  and  Beta — Radicles  of  Sinapis  aphelio- 
tropic,  due  to  the  sensitiveness  of  their  tips — Concluding  remarks 
and  summary  of  chapter — Means  by  which  circumnutation  has 
been  converted  into  heliotropism  or  apheliotropisrn  Page  449-492 


CHAPTER    X. 

MODIFIED-  CIRCUMNCTATION  :  MOVEMENTS  EXCITED  BY 
GRAVITATION. 

Means  of  observation — Apogeotropism — Cytisus — Verbena — Beta 
— Gradual  conversion  of  tlie  movement  of  circumnutation  into 
apogeotropism  in  Rubus,  Lilium,  Phalaris,  Avena,  and  Brassica 
— Apogeotropism  retarded  by  heliotropism — Effected  by  the  aid 
of  joints  or  pulvini — Movements  of  flower-peduncles  of  Oxalis — 
General  remarks  on  apogeotropism — Geotropism — Movements  of 
radicles — Burying  of  seed-capsules — Use  of  process — Trifolium 
subterraneum  —  Arachis  —  Amphicarpasa  —  Diageotropism  — 
Conclusion  493-522 


CHAPTER    XL 

LOCALISED  SENSITIVENESS  TO  GRAVITATION,  AND  ITS  TRANSMITTED 
EFFECTS. 

General  considerations — Vicia  faba,  effects  of  amputating  the  tips  of 
the  radicles — Regeneration  of  the  tips — Effects  of  a  short  ex- 
posure of  the  tips  to  geotropic  action  and  their  subsequent 
amputation — Effects  of  amputating  the  tips  obliquely — Effects 
of  cauterising  the  tips — Effects  of  grease  on  the  tips — Pisum 


CONTENTS. 

sativnm,  tips  of  radicles  cauterised  transversely,  and  on  their 
upper  and  lower  sides— Phaseolus,  cauterisation  and  grease  on 
the  tips — Gossypium — Cucurbita,  tips  cauterised  transversely, 
and  on  their  upper  and  lower  sides — Zea,  tips  cauterised — Con- 
cluding remarks  and  summary  of  chapter — Advantages  of  the 
sensibility  to  geotropism  being  localised  in  the  tips  of  the 
radicles Page  23-545 


CHAPTEE    XII. 

SUMMABY   AKD    CONCLUDING    REMARKS. 

Nature  of  the  circum nutating  movement — History  of  a  germinating 
seed — The  radicle  first  protrudes  and  circunmutates — Its  tip 
highly  sensitive — Emergence  of  the  hypocotyl  or  of  the  epicotyl 
from  the  ground  under  the  form  of  an  arch — Its  circumuutation 
and  that  of  the  cotyledons — The  seedling  throws  up  a  leaf- 
bearing  stem — The  circumnutation  of  all  the  parts  or  organs — 
Modified  circumnutation — Epinasty  and  hyponasty — Movements 
of  climbing  plants — Nyctitropic  movements — Movements  excited 
by  light  and  gravitation — Localised  sensitiveness — Resemblance 
between  the  movements  of  plants  and  animals — The  tip  of  the 
radicle  acts  like  a  brain  .. 546-573 

INDEX  .,  .,     574-593 


ERRATA. 


Page    3,  foot-note,  for  Mr.  Vine's  read  Mr.  Vines'. 
„  452,  line  7  from  bottom,  for  minntes  read  minutes. 


THE  MOVEMENTS  OF  PLANTS. 


INTRODUCTION. 

THE  chief  object  of  the  present  work  is  to  describe 
and  connect  together  several  large  classes  of  move- 
ment, common  to  almost  all  plants.  The  most  widely 
prevalent  movement  is  essentially  of  the  same  nature 
as  that  of  the  stem  of  a  climbing  plant,  which  bends 
successively  to  all  points  of  the  compass,  so  that  the 
tip  revolves.  This  movement  has  been  called  by 
Sachs  "  revolving  nutation ;"  but  we  have  found  it 
much  more  convenient  to  use  the  terms  circumnutation 
and  circumnutate.  As  we  shall  have  to  say  much 
about  this  movement,  it  will  be  useful  here  briefly  to 
describe  its  nature.  If  we  observe  a  circumnutating 
stem,  which  happens  at  the  time  to  be  bent,  we  will 
say  towards  the  north,  it  will  be  found  gradually  to 
bend  more  and  more  easterly,  until  it  faces  the  east ; 
and  so  onwards  to  the  south,  then  to  the  west,  and 
back  again  to  the  north.  If  the  movement  had  been 
quite  regular,  the  apex  would  have  described  a  circle, 
or  rather,  as  the  stem  is  always  growing  upwards,  a 
circular  spiral.  But  it  generally  describes  irregular 
elliptical  or  oval  figures ;  for  the  apex,  after  point- 
ing in  any  one  direction,  commonly  moves  back 
to  the  opposite  side,  not,  however,  returning  along 
the  same  line.  Afterwards  other  irregular  ellipses 
or  ovals  are  successively  described,  with  their  longer 


£  INTRODUCTION. 

axes  directed  to  different  points  of  the  compass. 
Whilst  describing  such  figures,  the  apex  often  travels 
in  a  zigzag  line,  or  makes  small  subordinate  loops  or 
triangles.  In  the  case  of  leaves  the  ellipses  are 
generally  narrow. 

Until  recently  the  cause  of  all  such  bending  move- 
ments was  believed  to  be  due  to  the  increased  growth 
of  the  side  which  becomes  for  a  time  convex ;  that  this 
side  does  temporarily  grow  more  quickly  than  the 
concave  side  has  been  well  established  ;  but  De  Vries 
has  lately  shown  that  such  increased  growth  follows 
a  previously  increased  state  of  turgescence  on  the 
convex  side.*  In  the  case  of  parts  provided  with  a 
so-called  joint,  cushion  or  pulvinus,  which  consists  of 
an  aggregate  of  small  cells  that  have  ceased  to 
increase  in  size  from  a  very  early  age,  we  meet  with 
similar  movements;  and  here,  as  Pfeffer  has  shown f 
and  as  we  shall  see  in  the  course  of  this  work, 
the  increased  turgescence  of  the  cells  on  opposite 
sides  is  not  followed  by  increased  growth.  Wiesner 
denies  in  certain  cases  the  accuracy  of  De  Tries'  con- 
clusion about  turgescence,  and  maintains  {  that  the 
increased  extensibility  of  the  cell-walls  is  the  more 
important  element.  That  such  extensibility  must 
accompany  increased  turgescence  in  order  that  the  part 
may  bend  is  manifest,  and  this  has  been  insisted  on  by 
several  botanists  ;  but  in  the  case  of  unicellular  plants 
it  can  hardly  fail  to  be  the  more  important  element. 
On  the  whole  we  may  at  present  conclude  that  in- 


*  Sachs  first   showed   CLehr-  19,  1879,  p.  830. 

buch,'  &c.,  4th  edit.  p.  452)  the  t  '  Die  Periodischen  Beweguu- 

intimate  connection  between  tur-  gen  der  Blattorgane,'  1875. 

gescence  and   growth.     For  De  £  *  Untersuchungen    iiber    den 

Vries'  interesting  essay,  '  Wachs-  Heliotropismus,'    Sitzb.    der    K. 

thumskriimmungen   mehrzelliger  Akad.derWissenschaft.  (Vienna), 

Organe,'  see  '  Dot.  Zeitung,'  Dec.  Jan.  1880. 


INTKODUCTION.  3 

creased  growth,  first  on  one  side  and  then  on  another, 
is  a  secondary  effect,  and  that  the  increased  tur- 
gescence  of  the  cells,  together  with  the  extensibility 
of  their  walls,  is  the  primary  cause  of  the  movement  of 
circumnutation.* 

In  the  course  of  the  present  volume  it  will  be  shown 
that  apparently  every  growing  part  of  every  plant  is 
continually  circumnutating,  though  often  on  a  small 
scale.  Even  the  stems  of  seedlings  before  they  have 
broken  through  the  ground,  as  well  as  their  buried 
radicles,  circum  nutate,  as  far  as  the  pressure  of  the 
surrounding  earth  permits.  In  this  universally  pre- 
sent movement  we  have  the  basis  or  groundwork  for 
the  acquirement,  according  to  the  requirements  of  the 
plant,  of  the  most  diversified  movements.  Thus,  the 
great  sweeps  made  by  the  stems  of  twining  plants, 
and  by  the  tendrils  of  other  climbers,  result  from 
a  mere  increase  in  the  amplitude  of  the  ordinary 
movement  of  circumnutation.  The  position  which 
young  leaves  and  other  organs  ultimately  assume 
is  acquired  by  the  circumnutating  movement  being 
increased  in  some  one  direction.  The  leaves  of 
various  plants  are  said  to  sleep  at  night,  and  it  will 
be  seen  that  their  blades  then  assume  a  vertical 
position  through  modified  circumnutation,  in  order 
to  protect  their  upper  surfaces  from  being  chilled 
through  radiation.  The  movements  of  various  organs 
to  the  light,  which  are  so  general  throughout  the 
vegetable  kingdom,  and  occasionally  from  the  light, 
or  transversely  with  respect  to  it,  are  all  modified 

*  See  Mr.  Vine's  excellent  dis-  Naturkunde  in  Wiirtemberg,' 
cussion  ('  Arbeitcn  des  Bot.  Insti-  1874,  p.  211)  on  the  curious  move- 
tuts  in  Wiirzburg,'  B.  II.  pp.  142,  ments  of  Spirogyra,  a  plant  con- 
143, 1878)  on  this  intricate  subject.  sisting  of  a  single  row  of  cells,.aro 
Hofmeister's  observations  (*  Jah-  valuable  in  relation  to  this  subject, 
reschrifte  des  Vereins  fur  Vaterl. 


4  INTRODUCTION. 

forms  of  circuumutation ;  as  again  are  the  equally 
prevalent  movements  of  stems,  &c.,  towards  the  zenith, 
and  of  roots  towards  the  centre  of  the  earth.  In 
accordance  with  these  conclusions,  a  considerable  diffi- 
culty in  the  way  of  evolution  is  in  part  removed,  for 
it  might  have  been  asked,  how  did  all  their  diversified 
movements  for  the  most  different  purposes  first  arise  ? 
As  the  case  stands,  we  know  that  there  is  always 
movement  in  progress,  and  its  amplitude,  or  direc- 
tion, or  both,  have  only  to  be  modified  for  the  good 
of  the  plant  in  relation  with  internal  or  external 
stimuli. 

Besides  describing  the  several  modified  forms  of 
circumnutation,  some  other  subjects  will  be  discussed. 
The  two  which  have  interested  us  most  are,  firstly,  the 
fact  that  with  some  seedling  plants  the  uppermost 
part  alone  is  sensitive  to  light,  and  transmits  an  influ- 
ence to  the  lower  part,  causing  it  to  bend.  If  there- 
fore the  upper  part  be  wholly  protected  from  light, 
the  lower  part  may  be  exposed  for  hours  to  it,  and  yet 
does  not  become  in  the  least  bent,  although  this  would 
have  occurred  quickly  if  the  upper  part  had  been 
excited  by  light.  Secondly,  with  the  radicles  of  seed- 
lings, the  tip  is  sensitive  to  various  stimuli,  espe- 
cially to  very  slight  pressure,  and,  when  thus  excited, 
transmits  an  influence  to  the  upper  part,  causing  it  to 
bend  from  the  pressed  side.  On  the  other  hand,  if 
the  tip  is  subjected  to  the  vapour  of  water  proceeding 
from  one  side,  the  upper  part  of  the  radicle  bends 
towards  this  side.  Again  it  is  the  tip,  as  stated  by 
Ciesielski,  though  denied  by  others,  which  is  sensitive 
to  the  attraction  of  gravity,  and  by  transmission  causes 
the  adjoining  parts  of  the  radicle  to  bend  towards  the 
centre  of  the  earth.  These  several  cases  of  the  effects 
of  contact,  other  irritants,  vapour,  light,  and  the 


INTRODUCTION.  5 

attraction  of  gravity  being  transmitted  from  the  ex- 
cited part  for  some  little  distance  along  the  organ  in 
question,  have  an  important  bearing  on  the  theory  of 
all  such  movements. 

Terminology. — A  brief  explanation  of  some  terms  which  will 
be  used,  must  here  be  given.  With  seedlings,  the  stem  which 
supports  the  cotyledons  (i.e.  the  organs  which  represent  the  first 
leaves)  has  been  called  by  many  botanists  the  hypocotyledonous 
stem,  but  for  brevity  sake  we  will  speak  of  it  merely  as  the 
hypocotyl:  the  stem  immediately  above  the  cotyledons  will  be 
called  the  e-ptcofyl  or  plumule.  The  radicls  can  be  distinguished 
from  the  hypocotyl  only  by  the  presence  of  root-hairs  and  the 
nature  of  its  covering.  The  meaning  of  the  word  circumnu- 
tatlon  has  already  been  explained.  Authors  speak  of  positive 
and  negative  heliotropism,*— that  is,  the  bending  of  an  organ 
to  or  from  the  light ;  but  it  is  much  more  convenient  to  confine 
the  word  hdiutropism  to  bending  towards  the  light,  and  to 
designate  as  a^heUutropism  bending  from  the  light.  There  is 
another  reason  for  this  change,  for  writers,  as  we  have 
observed,  occasionally  drop  the  adjectives  positive  and  negative, 
and  thus  introduce  confusion  into  their  discussions.  Diaheliv- 
tropism  may  express  a  position  more  or  less  transverse  to 
the  light  and  induced  by  it.  In  like  manner  positive  geotro- 
pism,  or  bending  towards  the  centre  of  the  earth,  will  bo 
called  by  us  geotropism ;  apogeotropi*m  will  mean  bending  in 
opposition  to  gravity  or  from  the  centre  of  the  earth ;  and  dia- 
geotropism,  a  position  more  or  less  transverse  to  the  radius  of 
the  earth.  The  words  heliotropism  and  geotropism  properly 
mean  the  act  of  moving  in  relation  to  the  light  or  the  earth ; 
but  in  the  same  manner  as  gravitation,  though  defined  as  "  the 
act  of  tending  to  the  centre,"  is  often  used  to  express  the  cause 
of  a  body  falling,  so  it  will  be  found  convenient  occasionally  to 
employ  heliotropism  and  geotropism,  &c.,  as  the  cause  of  the 
movements  in  question. 

The  term  epinasty  is  now  often  used  in  Germany,  and  implies 
that  the  upper  surface  of  an  organ  grows  more  quickly  than  the 


*  The  highly  useful  terras  of  Frank  :  see  his  remarkable  '  Bei- 
Heliotropism  and  Geotropism  trage  zur  Pflanzenphysiologie/ 
were  first  used  by  Dr.  A.  B.  1868. 


C  INTRODUCTION. 

lower  surface,  and  thus  causes  it  to  bend  downwards.  Hyi>&- 
nasty  is  the  reverse,  and  implies  increased  growth  along  the 
lower  surface,  causing  the  part  to  bend  upwards.* 

Methods  of  Observation. — The  movements,  sometimes  very 
small  and  sometimes  considerable  in  extent,  of  the  various 
organs  observed  by  us,  were  traced  in  the  manner  which  after 
many  trials  we  found  to  be  best,  and  which  must  be  described. 
Plants  growing  in  pots  were  protected  wholly  from  the  light, 
or  had  light  admitted  from  above,  or  on  one  side  as  the  case 
might  require,  and  were  covered  above  by  a  large  horizontal 
sheet  of  glass,  and  with  another  vertical  sheet  on  one  side.  A 
glass  filament,  not  thicker  than  a  horsehair,  and  from  a  quarter 
to  three-quarters  of  an  inch  in  length,  was  affixed  to  the  part  to 
be  observed  by  means  of  shellac  dissolved  in  alcohol.  The 
solution  was  allowed  to  evaporate,  until  it  became  so  thick  that 
it  set  hard  in  two  or  three  seconds,  and  it  never  injured  the 
tissues,  even  the  tips  of  tender  radicles,  to  which  it  was  applied. 
To  tho  end  of  the  glass  filament  an  excessively  minute  bead  of 
black  sealing-wax  was  cemented,  below  or  behind  which  a  bit  of 
card  with  a  black  dot  was  fixed  to  a  stick  driven  into  ths  ground. 
The  weight  of  the  filament  was  so  slight  that  even  small  leaves 
were  not  perceptibly  pressed  down.  Another  method  of  obser- 
vation, when  much  magnification  of  the  movement  was  not 
required,  will  presently  be  described.  The  bead  and  the  dot 
on  the  card  were  viewed  through  the  horizontal  or  vertical 
glass-plate  (according  to  the  position  of  the  object),  and  when 
one  exactly  covered  the  other,  a  dot  was  made  on  the  glass-plate 
with  a  sharply  pointed  stick  dipped  in  thick  Indian-ink.  Other 
dots  were  made  at  short  intervals  of  time  and  these  were  after- 
wards joined  by  straight  lines.  The  figures  thus  traced  were 
therefore  angular;  but  if  dots  had  been  made  every  1  or 
2  minutes,  the  lines  would  have  been  more  curvilinear,  as 
occurred  when  radicles  were  allowed  to  trace  their  own 
courses  on  smoked  glass-plates.  To  make  the  dots  accurately 
was  the  sole  difficulty,  and  required  some  practice.  Nor  could 
this  be  done  quite  accurately,  when  the  movement  was  much 
magnified,  such  as  30  times  and  upwards;  yet  even  in  this 
case  the  general  course  may  be  trusted.  To  test  the  accuracy 
of  the  above  method  of  observation,  a  filament  was  fixed  to  an 


*  These  terms  are  used  in  the      '  Wiirzburg    Arbeiten,'    Heft    ii. 
sense  given  them  by  De  Vries,       1872,  p.  252. 


INTRODUCTION.  7 

inanimate  object  which  was  made  to  snde  along  a  straight 
edge  and  dots  were  repeatedly  made  on  a  glass-plate;  when 
these  were  joined,  the  result  ought  to  have  been  a  perfectly 
straight  line,  and  the  line  was  very  nearly  straight.  It  may  be 
added  that  when  the  dot  on  the  card  was  placed  half-an-inch 
below  or  behind  the  bead  of  sealing-wax,  and  when  the  glass- 
plate  (supposing  it  to  have  been  properly  curved)  stood  at  a 
distance  of  7  inches  in  front  (a  common  distance),  then  the 
tracing  represented  the  movement  of  the  bead  magnified  15 
times. 

Whenever  a  great  increase  of  the  movement  was  not  required, 
another,  and  in  some  respects  better,  method  of  observation  was 
followed.  This  consisted  in  fixing  two  minute  triangles  of  thin 
paper,  about  ^  inch  in  height,  to  the  two  ends  of  the  attached 
glass  filament ;  and  when  their  tips  were  brought  into  a  line  so 
that  they  covered  one  another,  dots  were  made  as  before  on  the 
glass-plate.  If  we  suppose  the  glass-plate  to  stand  at  a  dis- 
tance of  seven  inches  from  the  end  of  the  shoot  bearing  the 
filament,  the  dots  when  joined,  will  give  nearly  the  same  figure 
as  if  a  filament  seven  inches  long,  dipped  in  ink,  had  been 
fixed  to  the  moving  shoot,  and  had  inscribed  its  own  course 
on  the  plate.  The  movement  is  thus  considerably  magnified; 
for  instance,  if  a  shoot  one  inch  in  length  were  bending,  and 
the  glass-plate  stood  at  the  distance  of  seven  inches,  the  move- 
ment would  be  magnified  eight  times.  It  would,  however,  have 
been  very  difficult  to  have  ascertained  in  each  case  how  great 
a  length  of  the  shoot  was  bending;  and  this  is  indispensable 
for  ascertaining  the  degree  to  which  the  movement  is  magnified. 

After  dots  had  been  made  on  the  glass-plates  by  either  of 
the  above  methods,  they  were  copied  on  tracing  paper  and 
joined  by  ruled  lines,  with  arrows  showing  the  direction  of  the 
movement.  The  nocturnal  courses  are  represented  by  straight 
broken  lines.  The  first  dot  is  always  made  larger  than  the 
others,  so  as  to  catch  the  eye,  as  may  be  seen  in  the  diagrams. 
The  figures  on  the  glass-plates  were  often  drawn  on  too  large 
a  scale  to  be  reproduced  on  the  pages  of  this  volume,  and  the 
proportion  in  which  they  have  been  reduced  is  always  given.* 
Whenever  it  could  be  approximately  told  how  much  the  move- 
ment had  been  magnified,  this  is  stated.  We  have  perhaps 


*  We    are  much   indebted   to       he  has  reduced  and  engraved  our 
Mr.  Cooper  for  the  care  with  which       diagrams. 


8  INTRODUCTION. 

introduced  a  superfluous  number  of  diagrams;  but  they  take 
up  less  space  than  a  full  description  of  the  movements.  Almost 
all  the  sketches  of  plants  asleep,  &c.,  were  carefully  drawn 
for  us  by  Mr.  George  Darwin. 

As  shoots,  leaves,  &c.,  in  circumnutating  bend  more  and 
more,  first  in  one  direction  and  then  in  another,  they  were 
necessarily  viewed  at  different  times  more  or  less  obliquely ; 
and  as  the  dots  were  made  on  a  flat  surface,  the  apparent 
amount  of  movement  is  exaggerated  according  to  the  degree 
of  obliquity  of  the  point  of  view.  It  would,  therefore,  have 
been  a  much  better  plan  to  have  used  hemispherical  glasses, 
if  we  had  possessed  them  of  all  sizes,  and  if  the  bending  part 
of  the  shoot  had  been  distinctly  hinged  and  could  have  been 
placed  so  as  to  have  formed  one  of  the  radii  of  the  sphere- 
But  even  in  this  case  it  would  have  been  necessary  afterwards 
to  have  projected  the  figures  on  paper;  so  that  complete 
accuracy  could  not  have  been  attained.  From  the  distortion 
of  our  figures,  owing  to  the  above  causes,  they  are  of  no  use 
to  any  one  who  wishes  to  know  the  exact  amount  of  movement, 
or  the  exact  course  pursued;  but  they  serve  excellently  for 
ascertaining  whether  or  not  the  part  moved  at  all,  as  well  as 
the  general  character  of  the  movement. 


In  the  following  chapters,  the  movements  of  a  con- 
siderable number  of  plants  are  described ;  and  the 
species  have  been  arranged  according  to  the  system 
adopted  by  Hooker  in  Le  Maout  and  Decaisne's  '  De- 
scriptive Botany.'  JSTo  one  who  is  not  investigating 
the  present  subject  need  read  all  the  details,  which, 
however,  we  have  thought  it  advisable  to  give.  To 
save  the  reader  trouble,  the  conclusions  and  most  of 
the  more  important  parts  have  been  printed  in  larger 
type  than  the  other  parts.  He  may,  if  he  thinks  fit, 
read  the  last  chapter  first,  as  it  includes  a  summary 
of  the  whole  volume ;  and  he  will  thus  see  what 
points  interest  him,  and  on  which  he  requires  the 
full  evidence. 

Finally,  we  must  have  the  pleasure  of  returning  our 


INTRODUCTION.  9 

sincere  thanks  to  Sir  Joseph  Hooker  and  to  Mr.  W. 
Thiselton  Dyer  for  their  great  kindness,  in  not  only 
sending  us  plants  from  Kew,  but  in  procuring  others 
from  several  sources  when  they  were  required  for  our 
observations  ;  also,  for  naming  many  species,  and  giving 
us  information  on  various  points. 


10  CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  L 


CHAPTER  I. 

THE    ClRCCMNUTATING   MOVEMENTS   OF   SEEDLING   PLANTS. 

Bras&ica  oleracea,  circummitation  of  the  radicle,  of  the  arched  hypo- 
cotyl  whilst  still  buried  beneath  the  ground,  whilst  rising  above  the 
ground  and  straightening  itself,  and  when  erect — Circummitation 
of  the  cot}  ledons — Rate  of  movement — Analogous  observations  on 
various  organs  in  species  of  Githago,  Gossypium,  Oxalis,  Tro- 
paeolum,  Citrus,  ^sculus,  of  several  Leguminous  and  Cucurbita- 
ceous  genera,  Opuntia,  Helianthus,  Primula,  Cyclamen,  Stapel'a, 
Cerinthe,  Nolana,  Solanum,  Beta,  Ricinus,  Quercus,  Corylus,  Pinus, 
Cycas,  Canna,  Allium,  Asparagus,  Phalaris,  Zea,  Avena,  Nephro- 
cliuiu,  and  Selagiuella. 

THE  following  chapter  is  devoted  to  the  circum- 
nutating  movements  of  the  radicles,  hypocotyls,  and 
cotyledons  of  seedling  plants ;  and,  when  the  coty- 
ledons do  not  rise  above  the  ground,  to  the  movements 
of  the  epicotyl.  But  in  a  future  chapter  we  shall  have 
to  recur  to  the  movements  of  certain  cotyledons  which 
sleep  at  night. 

Brassica  oleracea  (Cructferce). — Fuller  details  will  be  given 
with  respect  to  the  movements  in  this  case  than  in  any  other, 
as  space  and  time  will  thus  ultimately  be  saved. 

JRadide. — A  seed  with  the  radicle  projecting  -05  inch  was 
fastened  with  shellac  to  a  little  plate  of  zinc,  so  that  the 
radicle  stood  up  vertically;  and  a  fine  glass  filament  was  then 
fixed  near  its  base,  that  is,  close  to  the  seed- coats.  The  seed 
was  surrounded  by  little  bits  of  wet  sponge,  and  the  move- 
ment of  the  bead  at  the  end  of  the  filament  was  traced  (Fig.  1) 
during  sixty  hours.  In  this  time  the  radicle  increased  in 
length  from  '05  to  '11  inch.  Had  the  filament  been  attached  at 
first  close  to  the  apex  of  the  radicle,  and  if  it  could  have  re- 
mained there  all  the  time,  the  movement  exhibited  would  have 


CHAP.  I, 


BRASSICA. 


11 


Fig.  1. 


been  much  greater,  for  at  the  close  of  our  observations  the  tip, 
instead  of  standing  vertically  upwards,  had  become  bowed 
downwards  through  geotropism,  so  as  almost  to  touch  the  zinc 
plate.  As 'far  as  we  could 
roughly  ascertain  by  measure- 
ments made  with  compasses 
on  other  seeds,  the  tip  alone, 
for  a  length  of  only  y§o  to 
T£o  of  an  inch,  is  acted  on 
by  geotropism.  But  the  trac- 
ing shows  that  the  basal  part 
of  the  radicle  continued  to 
cireumnutate  irregularly  dur- 
ing the  whole  time.  The 
actual  extreme  amount  of 
movement  of  the  bead  at  the 
end  of  the  filament  was  nearly 
•05  inch,  but  to  what  extent 
the  movement  of  the  radicle 
was  magnified  by  the  fila- 
ment, which  was  nearly  I  inch 
in  length,  it  was  impossible 
to  estimate. 

Another  seed  was  treated  and  observed  in  the  same  manner, 
but  the  radicle  in  this  case  protruded  '1  inch,  and  was  not 


Fig.  2. 


Brassca  oleriacea  :  circumnutation  of 
radicle,  traced  on  horizontal  glass, 
from  9  A.M.  Jan.  31st  to  9  P.M. 
Feb.  2nd.  Movement  of  bead  at 
end  of  filament  magnified  about 
40  times. 


\ 

Brassica  oleracea  :  oircumnutating  and  geotropic  movement  of  radicle, 
traced  on  horizontal  glass  during  46  hours. 

fastened  so  as  to  project  quite  vertically  upwards.  The  filament 
was  affixed  close  to  its  base.  The  tracing  (Fig.  2,  reduced  by 
half)  shows  the  movement  from  9  A.M.  Jan.  31st  to  7  A. AT. 
Feb.  2nd;  but  it  continued  to  move  during  the  whole  of  tho 


12  CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 

'2nd  in  the  same  general  direction,  and  in  a  similar  zigzag 
manner.  From  the  radicle  not  being  quite  perpendicular  when 
the  filament  was  affixed  geotropism  came  into  play  at  once; 
but  the  irregular  zigzag  course  shows  that  there  was  growth 
(probably  preceded  by  turgescence),  sometimes  on  one  and 
sometimes  on  another  side.  Occasionally  the  bead  remained 
stationary  for  about  an  hour,  and  then  probably  growth  occurred 
on  the  side  opposite  to  that  which  caused  the  geotropic  curva- 
ture. In  the  case  previously  described  the  basal  part  of  the 
very  short  radicle  from  being  turned  vertically  upwards,  was  at 
first  very  little  affected  by  geotropism.  Filaments  were  affixed 
in  two  other  instances  to  rather  longer  radicles  protruding 
obliquely  from  seeds  which  had  been  turned  upside  down ;  and 
in  these  cases  the  lines  traced  on  the  horizontal  glasses  were 
only  slightly  zigzag,  and  the  movement  was  always  in  the  same 
general  direction,  through  the  action  of  geotropism.  All  these 
observations  are  liable  to  several  causes  of  error,  but  we  believe, 
from  what  will  hereafter  be  shown  with  respect  to  the  move- 
ments of  the  radicles  of  other  plants,  that  they  may  be  largely 
trusted. 

Hypocotyl. — The  hypocotyl  protrudes  through  the  seed-coats 
as  a  rectangular  projection,  which  grows  rapidly  into,  an  arch 
like  the  letter  U  turned  upside  down  f]  5  the  cotyledons  being 
still  enclosed  within  the  seed.  In  whatever  position  the  seed 
may  be  embedded  in  the  earth  or  otherwise  fixed,  both  legs  of 
the  arch  bend  upwards  through  apogeotropism,  and  thus  rise 
vertically  above  the  ground.  As  soon  as  this  has  taken  place, 
or  even  earlier,  the  inner  or  concave  surface  of  the  arch  grows 
more  quickly  than  the  upper  or  convex  surface;  and  this  tends 
to  separate  the  two  legs  and  aids  in  drawing  the  cotyledons  out 
of  the  buried  seed-coats.  By  the  growth  of  the  whole  arch  the 
cotyledons  are  ultimately  dragged  from  beneath  the  ground,  even 
from,  a  considerable  depth;  and  now  the  hypocotyl  quickly 
straightens  itself  by  the  increased  growth  of  the  concave  side. 

Even  whilst  the  arched  or  doubled  hypocotyl  is  still  beneath 
the  ground,  it  circumnutates  as  much  as  the  pressure  of  the  sur- 
rounding soil  will  permit;  but  this  was  difficult  to  observe, 
because  as  soon  as  the  arch  is  freed  from  lateral  pressure  the  two 
legs  begin  to  separate,  even  at  a  very  early  age,  before  the  arch 
would  naturally  have  reached  the  surface.  Seeds  were  allowed 
to  germinate  on  the  surface  of  damp  earth,  and  after  they  had 
fixed  themselves  by  their  radicles,  and  after  the,  as  yet,  only 


CHAP.  I.  BEASSICA.  13 

slightly  arched  hypocotyl  had  become  nearly  vertical,  a  glass 
filament  was  affixed  on  two  occasions  near  to  the  base  of  the 
basal  leg  (i.e.  the  one  in  connection  with  the  radicle),  and  its 
movements  were  traced  in  darkness  on  a  horizontal  glass.  The 
result  was  that  long  lines  were  formed  running  in  nearly  the 
plane  of  the  vertical  arch,  due  to  the  early  separation  of  the 
two  legs  now  freed  from  pressure ;  but  as  the  lines  were  zigzag, 
showing  lateral  movement,  the  arch  must  have  been  circum- 
nutating,  whilst  it  was  straightening  itself  by  growth  along  its 
inner  or  concave  surface. 
A  somewhat  different  method  of  observation  was  next  followed : 

Fig.  3. 


Brassica  oleracea :  circumnutating  movement  of  buried  and  arched  hypo- 
cotvl  (dimly  illuminated  from  above),  traced  on  horizontal  glass  during 
45  hours.  Movement  of  bead  of  filament  magnified  about  25  times, 
and  here  reduced  to  one-half  of  original  scale. 

as  soon  as  the  earth  with  seeds  in  a  pot  began  to  crack,  the 
surface  was  removed  in  parts  to  the  depth  of  '2  inch ;  and  a 
filament  was  fixed  to  the  basal  leg  of  a  buried  and  arched  hypo- 
cotyl, just  above  the  summit  of  the  radicle.  The  cotyledons 
were  still  almost  completely  enclosed  within  the  much-cracked 
seed-coats ;  and  these  were  again  covered  up  with  damp  adhesive 
soil  pressed  pretty  firmly  down.  The  movement  of  the  filament 
was  traced  (Fig.  3)  from  11  A.M.  Feb.  5th  till  8  A.M.  Feb.  7th. 
By  this  latter  period  the  cotyledons  had  been  dragged  from 
beneath  the  pressed-down  earth,  but  the  upper  part  of  the 
hypocotyl  still  formed  nearly  a  right  angle  with  the  lower  part. 
The  tracing  shows  that  the  arched  hypocotyl  tends  at  this  early 


M  CIKCUMNUTATION   OF  SEEDLINGS.         CHAT.  L 

age  to  circumnutate  irregularly.  On  the  first  day  the  greater 
movement  (from  right  to  left  in  the  figure)  was  not  in  the  plane 
of  the  vertical  and  arched  hypocotyl,  but  at  right  angles  to  it,  or  in 
the  plane  of  the  two  cotyledons,  which  were  still  in  close  contact. 
The  basal  leg  of  the  arch  at  the  time  when  the  filament  was 
affixed  to  it,  was  already  bowed  considerably  backwards,  or 
from  the  cotyledons ;  had  the  filament  been  affixed  before  this 
bowing  occurred,  the  chief  movement  would  have  been  at  right 
angles  to  that  shown  in  the  figure.  A  filament  was  attached  to 
another  buried  hypocotyl  of  the  same  age,  and  it  moved  in  a 
similar  general  manner,  but  the  line  traced  was  not  so  complex. 
This  hypocotyl  became  almost  straight,  and  the  cotyledons  were 
dragged  from  beneath  the  ground  on  the  evening  of  the  second  day. 

Fig.  4. 


Brassica  oleracea :  circumnutating  movement  of  buried  and  arched  hypo- 
cotyl, with  the  two  legs  of  the  arch  tied  together,  traced  on  horizontal 
glass  during  33J  hours.  Movement  of  the  bead  of  filament  magnified 
about  26  times,  and  here  reduced  to  one-half  original  scale. 

Before  the  above  observations  were  made,  some  arched  hypo- 
cotyls  buried  at  the  depth  of  a  quarter  of  an  inch  were  un- 
covered; and  in  order  to  prevent  the  two  legs  of  the  arch 
from  beginning  to  separate  at  once,  they  were  tied  together  with 
fine  silk.  This  was  done  partly  because  we  wished  to  ascertain 
how  long  the  hypocotyl,  in  its  arched  condition,  would  continue 
to  move,  and  whether  the  movement  when  not  masked  and 
disturbed  by  the  straightening  process,  indicated  circumnu- 
tation.  Firstly,  a  filament  was  fixed  to  the  basal  leg  of  an 
arched  hypocotyl  close  above  the  summit  of  the  radicle.  The 
cotyledons  were  still  partially  enclosed  within  the  seed-coats. 
The  movement  was  traced  (Fig.  4)  from  9.20  A.M.  on  Dec. 


CHAP.  I.  BRASSICA.  15 

'23rd  to  6.45  A.M.  on  Dec.  25th.  No  doubt  the  natural  move- 
ment was  much  disturbed  by  the  two  legs  having  been  tied 
together ;  but  we  see  that  it  was  distinctly  zigzag,  first  in  one 
direction  and  then  in  an  almost  opposite  one.  After  3  P.M.  on 
the  24th  the  arched  hypocotyl  sometimes  remained  stationary 
for  a  considerable  time,  and  when  moving,  moved  far  slower  than 
before.  Therefore,  on  the  morning  of  the  25th,  the  glass  fila- 
ment was  removed  from  the  base  of  the  basal  leg,  and  was  fixed 
horizontally  on  the  summit  of  the  arch,  which,  from  the  legs 
having  been  tied,  had  grown  broad  and  almost  flat.  The 
movement  was  now  traced  during  23  hours  (Fig.  5),  and  we 

Fig.  5. 


Bmssica  oleracea:  circurnmtating  movement  of  the  crown  of  a  buried  and 
arched  hypocotyl,  with  the  two  legs  tied  together,  traced  on  a  hori- 
zontal glass  during  23  hours.  Movement  of  the  bead  of  the  filament 
magnified  about  58  times,  and  here  reduced  to  one-half  original 
scale. 

see  that  the  course  was  still  zigzag,  which  indicates  a  tendency 
to  circumnutation.  The  base  of  the  basal  leg  by  this  time  had 
almost  completely  ceased  to  move. 

As  soon  as  the  cotyledons  have  been  naturally  dragged  from 
beneath  the  ground,  and  the  hypocotyl  has  straightened  itself 
by  growth  along  the  inner  or  concave  surface,  there  is  nothing  to 
interfere  with  the  free  movements  of  the  parts ;  and  the  circum- 
nutation now  becomes  much  more  regular  and  clearly  displayed, 
as  shown  in  the  following  cases :  — A  seedling  was  placed  in 
front  and  near  a  north-east  window  with  a  line  joining  the 
2 


16 


CIKCUMNUTATION   OF   SEEDLINGS.         CHAP.  I. 


two  cotyledons  parallel  to  the  window.  It  was  thus  left  the 
whole  day  so  as  to  accommodate  itself  to  the  light.  On  the 
following  morning  a  filament  was  fixed  to  the  midrib  of  the 
larger  and  taller  cotyledon  (which  enfolds  the  other  and  smaller 
one,  whilst  still  within  the  seed),  and  a  mark  being  placed 
close  behind,  the  movement  of  the  whole  plant,  that  is,  of  the 
hypocotyl  and  cotyledon,  was  traced  greatly  magnified  on  a  ver- 
tical glass.  At  first  the  plant  bent  so  much  towards  the  light 
that  it  was  useless  to  attempt  to  trace  the  movement ;  but  at 
10  A.M.  heliotropism  almost  wholly  ceased  and  the  first  dot  was 

Fig.  6. 


•« 

Brassica  oleracea :  conjoint  circummitation  of  the  hypocotyl  and  cotyledons 
during  10  hours  45  minutes.  Figure  here  reduced  to  one-half  original 
scale. 

made  on  the  glass.  The  last  was  made  at  8.45  P.M.;  seventeen 
dots  being  altogether  made  in  this  interval  of  10  h.  45  m.  (see 
Fig.  6).  It  should  be  noticed  that  when  I  looked  shortly  after 
4  P.M.  the  bead  was  pointing  off  the  glass,  but  it  came  on  again 
at  5.30  P.M.,  and  the  course  during  this  interval  of  1  h.  30  m.  has 
been  filled  up  by  imagination,  but  cannot  be  far  from  correct. 
The  bead  moved  seven  times  from  side  to  side,  and  thus  de- 
scribed 3s  ellipses  in  101  h. ;  each  being  completed  on  an 
average  in  3  h.  4  m. 

On  the  previous  day  another  seedling  had  been  observed 
under   similar  conditions,  excepting  that  the  plant   was    so 


CHAP.  I. 


BRASSICA. 


17 


placed  that  a  line  joining  the  two  cotyledons  pointed  towards 
the  window ;  and  the  filament  was  attached  to  the  smaller  Coty- 
ledon on  the  side  furthest  from  the  window.  Moreover,  the 
plant  was  now  for  the  first  time  placed  in  this  position.  The 
cotyledons  bowed  themselves  greatly  towards  the  light  from  8  to 
10.50  A.M.,  when  the  first  dot  was  made  (Fig.  7).  During  the 

Fig.  7. 


Brassica  oleracea  :  conjoint  circumnutation  of  the  hypocotyl  and  cotyledons, 
from  10.50  A.M.  to  8  A.M.  on  the  following  morning.  Tracing  made 
on  a  vertical  glass. 

next  12  hours  the  bead  swept  obliquely  up  and  down  8  times 
and  described  4  figures  representing  ellipses ;  so  that  it  travelled 
at  nearly  the  same  rate  as  in  the  previous  case.  During  the 
night  it  moved  upwards,  owing  to  the  sleep-movement  of  the 
cotyledons,  and  continued  to  move  in  the  same  direction  till 
9  A.M.  on  the  following  morning ;  but  this  latter  movement 
would  not  have  occurred  with  seedlings  under  their  natural 
conditions  fully  exposed  to  the  light. 
By  9.25  A.M.  on  this  second  day  the  same  cotyledon  had 


18 


CIRCUMNUTATION.OF   SEEDLINGS.         CHAP.  L 


Fig.  8. 


begun  to  fall,  and  a  dot  was  made  on  a  fresh  glass.  The  move- 
ment was  traced  until  5.30  P.M.  as  shown  in  (Fig.  8),  which  is 
given,  because  the  course  followed  was  much  more'  irregular 

than  on  the  two  previous 
occasions.  During  these 
8  hours  the  bead  changed 
its  course  greatly  10  times. 
The  upward  movement  of 
the  cotyledon  during  the 
afternoon  and  early  part 
of  the  night  is  here  plainly 
shown. 

As  the  filaments  were 
fixed  in  the  three  last 
cases  to  one  of  the  coty- 
ledons, and  as  the  hypo- 
cotyl  was  left  five,  the 
tracings  show  the  move- 

Brassica  oleracea  :  conjoint  circiramutation  ment  °f  ^^  organs  C°Q- 
of  the  hypocotyl  and  cotyledons  during  joined  ;  and  we  now 
8  hours.  Figure  here  reduced  to  one-  wished  to  ascertain  whe- 

ther  ^  circumnutated. 
Filaments  were  therefore 
fixed  horizontally  to  two  hypocotyls  close  beneath  the  petioles 
of  their  -cotyledons.  These  seedlings  had  stood  for  two  days 
in  the  same  position  before  a  north-east  window.  In  the  morn- 
ing, up  to  about  11  A.M.,  they  moved  in  zigzag  lines  towards 
the  light;  and  at  night  they  again  became  almost  upright 
through  apogeotropism.  After  about  11  A.M.  they  moved  a 
little  back  from  the  light,  often  crossing  and  recrossing  their 
former  path  in  zigzag  lines.  The  sky  on  this  day  varied  much 
in  brightness,  and  these  observations  merely  proved  that  the 
hypocotyls  were  continually  moving  in  a  manner  resembling 
circumnutation.  On  a  previous  day  which  was  uniformly 
cloudy,  a  hypocotyl  was  firmly  secured  to  a  little  stick,  and 
a  filament  was  fixed  to  the  larger  of  the  two  cotyledons,  and  its 
movement  was  traced  on  a  vertical  glass.  It  fell  greatly  from 
8.52  A.M.,  when  the  first  dot  was  made,  till  10.55  A.M.  ;  it  then  rose 
greatly  until  12.17  P.M.  Afterwards  it  fell  a  little  and  made  a 
loop,  but  by  2.22  P.M.  it  had  risen  a  little  and  continued  rising 
till  9.23  P.M.,  when  it  made  another  loop,  and  at  10.30  P.M.  was 
again  rising.  These  observations  show  that  the  cotyledons  move 


vertical 


CHAP.  I. 


BRASSICA. 


19 


Fig.  9. 


vertically  up  and  down  all  day  long,  and. as  there  was  some 
slight  lateral  movement,  they  circumnutated. 

The  cabbage  was  one  of  the  first  plants,  the  seedlings  of  which 
were  observed  by  us,  and  we 
did  not  then  know  how  far 
the  circummitation  of  the 
different  parts  was  aifected 
by  light.  Young  seedlings 
were  therefore  kept  in  com- 
plete darkness  except  for-  a 
minute  or  two  during  each 
observation,  when  they  were 
illuminated  by  a  small  wax 
taper  held  almost  vertically 
above  them.  During  the  first 
day  the  hypocotyi  of  one 
changed  its  course  18  times 
(see  Fig.  9) ;  and  it  deserves 
notice  that  the  longer  axes 
of  the  figures  described  often 
cross  one  another  at  right  or 
nearly  right  angles.  Another 
seedling  was  observed  in  the 
same  manner,  but  it  was 
much  older,  for  it  had  formed 
a  true  leaf  a  quarter  of  an 
inch  in  length,  and  the  hy- 
pocotyi was  If  inch  in  height. 
The  figure  traced  was  a  very 
complex  one,  though  the 
movement  was  not  so  great 
in  extent  as  in  the  last  case. 

The  hypocotyi  of  another 
seedling  of  the  same  age  was 
secured  to  a  little  stick,  and 
a  filament  having  been  fixed 
to  the  midrib  of  one  of  the 
cotyledons,  the  movement  of 
the  bead  was  traced  during  14  h.  15m.  (see  Fig.  10)  in  darkness. 
It  should  be  noted  that  the  chief  movement  of  the  cotyledons, 
namely,  up  and  down,  would  be  shown  on  a  horizontal  glass- 
plate  only  by  the  lines  in  the  direction  of  the  midrib  (that  is, 


Brassica  oleracea  :  circumnutation  of 
hypocotyi,  in  darkness,  traced  on  a 
horizontal  glass,  by  means  of  a  fila- 
ment with  a  bead  fixed  across  its 
summit,  between  9.15  A.M.  and 
8.30  A.M.  on  the  following  morn- 
ing. Figure  here  reduced  to  one- 
half  of  original  scale. 


20 


CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 


Fig.  10. 


up  and  down,  as  Fig.  10  here  stands)  being  a  little  lengthened 
or  shortened ;  whereas  any  -lateral  movement  would  be  well 
exhibited.  The  present  tracing  shows 
that  the  cotyledon  did  thus  move  laterally 
(that  is,  from  side  to  side  in  the  tracing) 
12  times  in  the  14  h.  15  m.  of  observa- 
tion. Therefore  the  cotyledons  certainly 
circumnutated,  though  the  chief  move- 
ment was  up  and  down  in  a  vertical 
plane. 

Rate  of  movement. — The  movements  of 
the  hypocotyls  and  cotyledons  of  seedling 
cabbages  of  different  ages  have  now  been 
sufficiently  illustrated.  With  respect  to 
the  rate,  seedlings  were  placed  under  the 
microscope  with  the  stage  removed,  and 
with  a  micrometer  eye-piece  so  adjusted 
that  each  division  equalled  5^  inch ;  the 
plants  were  illuminated  by  light  passing 
through  a  solution  of  bichromate  of  potas- 
sium so  as  to  eliminate  heliotropism. 
Under  these  circumstances  it  was  interest- 
ing to  observe  how  rapidly  the  circum- 
nutating  apex  of  a  cotyledon  passed  across 
the  divisions  of  the  micrometer.  Whilst 
travelling  in  any  direction  the  apex  generally  oscillated  back- 
wards and  forwards  to  the  extent  of  -^-^  and  sometimes  of  nearly 
•2^0  of  an  inch.  These  oscillations  were  quite  different  from  the 
trembling  caused  by  any  disturbance  in  the  same  room  or  by 
the  shutting  of  a  distant  door.  The  first  seedling  observed  was 
nearly  two  inches  in  height  and  had  been  etiolated  by  having 
been  grown  in  darkness.  The  tip  of  the  cotyledon  passed  across 
10  divisions  of  the  micrometer,  that  is,  -^  of  an  inch,  in  6  m. 
40  s.  Short  glass  filaments  were  then  fixed  vertically  to  the 
hypocotyls  of  several  seedlings  so  as  to  project  a  little  above  the 
cotyledons,  thus  exaggerating  the  rate  of  movement ;  but  only  a 
few  of  the  observations  thus  made  are  worth  giving.  The  most 
remarkable  fact  was  the  oscillatory  movement  above  described, 
and  the  difference  of  rate  at  which  the  point  crossed  the  divi- 
sions of  the  micrometer,  after  short  intervals  of  time.  For 
instance,  a  tall  not-etiolated  seedling  had  been  kept  for  14  h. 
in  darkness ;  it  was  exposed  before  a  north-east  window  for  only 


Brassica  ole.racca  :  cir- 
cunmutation  of  a 
cotyledon,  the  hypo- 
cotyl  having  been 
secured  to  a  stick, 
traced  on  a  horizon- 
tal glass,  in  dark- 
ness, from  8.15  A.M. 
to  10.30  P.M.  Move- 
ment of  the  bead  of 
the  filament  magni- 
fied 13  times. 


CHAP.  I.  GITHAGO.  21 

two  or  three  minutes  whilst  a  glass  filament  was  fixed  vertically 
to  the  hypocotyl ;  it  was  then  again  placed  in  darkness  for  half 
an  hour  and  afterwards  observed  by  light  passing  through 
bichromate  of  potassium.  The  point,  oscillating  as  usual, 
crossed  five  divisions  of  the  micrometer  (i.e.  y^  inch)  in 
1  m.  30  s.  The  seedling  was  then  left  in  darkness  for  an  hour, 
and  now  it  required  3m.  6s.  to  cross  one  division,  that  is, 
15  m.  30  s.  to  have  crossed  five  divisions.  Another  seedling, 
after  being  occasionally  observed  in  the  back  part  of  a  northern 
room  with  a  very  dull  light,  and  left  in  complete  darkness  for 
intervals  of  half  an  hour,  crossed  five  divisions  in  5  m.  in  the 
direction  of  the  window,  so  that  we  concluded  that  the  move- 
ment was  heliotropic.  But  this  was  probably  not  the  case,  for 
it  was  placed  close  to  a  north-east  window  and  left  there  for 
25  m.,  after  which  time,  instead  of  moving  still  more  quickly 
towards  the  light,  as  might  have  been  expected,  it  travelled 
only  at  the  rate  of  12  m.  30  s.  for  five  divisions.  It  was  then 
again  left  in  complete  darkness  for  lh.,  and  the  point  now 
travelled  in  the  same  direction  as  before,  but  at  the  rate  of 
3  m.  18  s.  for  five  divisions. 

We  shall  have  to  recur  to  the  cotyledons  of  the  cabbage  in  a 
future  chapter,  when  we  treat  of  their  sleep-movements.  The 
circumnutation,  also,  of  the  leaves  of  fully-developed  plants 
will  hereafter  be  described. 

Fig.  11. 


Githago  seyetum:  circumnutation  of  hypocotyl,  traced  on  a  horizontal 
glass,  by  means  of  a  filament  fixed  transversely  across  its  summit,  from 
8.15  A.M.  to  12.15  P.M.  on  the  following  day.  Movement  of  bead  of 
filament  magnified  about  13  times,  here  reduced  to  one-half  the  original 
scale. 

Githago  segetum  (Caryophylleae). — A  young  seedling  was  dimly 
illuminated  from  above,  and  the  circumnutation  of  the  hypo- 


22  CIKCUMNUTATION   OF  SEEDLINGS.         CHAP.  I. 

cotyl  was  observed  during  28  h.,  as  shown  in  Fig.  11.  It  moved 
in  all  directions;  the  lines  from  right  and  to  left  in  the  figure 
being  parallel  to  the  blades  of  the  cotyledons.  The  actual 
distance  travelled  from  side  to  side  by  the  summit  of  the 
hypocotyl  was  about  '2  of  an  inch;  but  it  was  impossible  to 
be  accurate  on  this  head,  as  the  more  obliquely  the  plant  was 
viewed,  after  it  had  moved  for  some  time,  the  more  the  distances 
were  exaggerated. 

We  endeavoured  to  observe  the  circumnutation  of  the  coty- 
ledons, but  as  they  close  together  unless  kept  exposed  to  a  mode- 
rately bright  light,  and  as  the  hypocotyl  is  extremely  heliotropic, 
the   necessary    arrangements    were    too 
troublesome.    We  shall  recur  to  the  noc- 
turnal or  sleep-movements  of  the  cotyle- 
dons in  a  future  chapter. 

Gossypium  (var.  Nankin  cotton)  (Mal- 
vaceae).— The  circumnutation  of  a  hypo- 
cotyl wTas  observed  in  the  hot-house,  but 
Goss'/pium .-  cn-cumnu-     the  movement  was  so  much  exaggerated 
tation  of  hypocotyl,      t] t  ^    b     d  twice          ed  for     ti  t    f 

traced  on  a  horizon- 
tal glass,  from  10.30     view.    It  was,  however,  manifest  that  two 
A.M.  to  9.30  A.M.  on     somewhat  irregular  ellipses  were  nearly 
following    morning,     completed   in   9   h.      Another   seedling, 
by  means  ot  a  nlu-      -,,•••>•-,,  ,1          •,  -IT- 

ment    fixed    across     H  m.  in  height,  was  then  observed  during 

its  summit.    Move-     23 h.;    but    the    observations   were    not 
ment  of  bead  of  fila-     made  at  sufficiently  short  "intervals,  as 

rwtTas?ltaiii"-  shown  by  the  few  dots  in  Kg- l2- and  the 

minated  from  above,  tracing  was  not  now  sufficiently  enlarged. 
Nevertheless  there  could  be  no  doubt 

about  the  circumnutation  of  the  hypocotyl,  which  described 
in  12  h.  a  figure  representing  three  irregular  ellipses  of  unequal 
sizes. 

The  cotyledons  are  in  constant  movement  up  and  down  during 
the  whole  day,  and  as  they  offer  the  unusual  case  of  moving 
downwards  late  in  the  evening  and  in  the  early  part  of  the 
night,  many  observations  were  made  on  them.  A  filament  was 
fixed  along  the  middle  of  one,  and  its  movement  traced  on  a 
vertical  glass;  but  the  tracing  is  not  given,  as  the  hypocotyl 
was  not  secured,  so  that  it  was  impossible  to  distinguish  clearly 
between  its  movement  and  that  of  the  cotyledon.  The  coty- 
ledons rose  from  10.30  A.M.  to  about  3  P.M.  ;  they  then  sank  till 
10  P.M.,  rising,  however,  greatly  in  the  latter  part  of  the  nigLt. 


CHAP.  I.  GOSSYPIUM.  23 

The  angles  above  the  horizon  at  which  the  cotyledons  of  another 
seedling  stood  at  different  hours  is  recorded  in  the  following 
short  table : — 

Oct.  20     2.50  P.M 25°  above  horizon. 

»        4.20   „  ..      22° 

5.20  „ 


„      10-40   „ 
Oct.  21     8.40  A.M. 
11.15   „ 


28° 
35° 


„        9.11  P.M 10°  below  horizon. 

The  position  of  the  two  cotyledons  was  roughly  sketched  at 
various  hours  with  the  same  general  result. 

In  the  following  summer,  the  hypocotyl  of  a  fourth  seedling 
was  secured  to  a  little  stick,  and  a  glass  filament  with  triangles 
of  paper  having  been  fixed  to  one  of  the  cotyledons,  its  move- 
ments were  traced  on  a  vertical  glass  under  a  double  skylight  in 
the  house.  The  first  dot  was  made  at  4.20  P.M.  June  20th ;  and 
the  cotyledon  fell  till  10.15  P.M.  in  a  nearly  straight  line.  Just 
past  midnight  it  was  found  a  little  lower  and  somewhat  to  one 
side.  By  the  early  morning,  at  3.45  A.M.,  it  had  risen  greatly, 
but  by  6.20  A.M.  had  fallen  a  little.  During  the  whole  of  this 
day  (21st)  it  fell  in  a  slightly  zigzag  line,  but  its  normal  course 
was  disturbed  by  the  want  of  sufficient  illumination,  for  during 
the  night  it  rose  only  a  little,  and  travelled  irregularly  during 
the  whole  of  the  following  day  and  night  of  June  22nd.  The 
ascending  and  descending  lines  traced  during  the  three  days 
did  not  coincide,  so  that  the  movement  was  one  of  circumnuta- 
tion.  This  seedling  was  then  taken  back  to  the  hot-house,  and 
after  five  days  was  inspected  at  10  P.M.,  when  the  cotyledons 
were  found  hanging  so  nearly  vertically  down,  that  they  might 
justly  be  said  to  have  been  asleep.  On  the  following  morning 
they  had  resumed  their  usual  horizontal  position. 

Oxalis  rosea  (Oxalidess). — The  hypocotyl  was  secured  to  a  little 
stick,  and  an  extremely  thin  glass  filament,  with  two  triangles  of 
paper,  was  attached  to  one  of  the  cotyledons,  which  was  '15  inch 
in  length.  In  this  and  the  following  species  the  end  of  the 
petiole,  where  united  to  the  blade,  is  developed  into  a  pulvinus. 
The  apex  of  the  cotyledon  stood  only  5  inches  from  the  vertical 
glass,  so  that  its  movement  was  not  greatly  exaggerated  as  long 
as  it  remained  nearly  horizontal ;  but  in  the  course  of  the  day  it 
both  rose  considerably  above  and  fell  beneath  a  horizontal  posi- 
tion, and  then  of  course  the  movement  was  much  exaggerated. 


1M 


TATION    01*' 


ClIAl'.     I. 


Ill   I'l;1;.   Ill  its   course  is   S!IONN  n    from    ('.  I.'.   A.M.  on   .lime    IVlh,  to 

7  l(l  \  M  on  t  he  I'olloNN  Hi".  morn- 
Hi!1  ;  Mild  NNC  see  (hat  dm  ni".  the 
da\  t  line,  in  the  course  of  I  I  h. 
!•>  in  .  it  1  ravelled  thrice  do\\  n 
Mild  t  NVICO  up.  Alter  ,r>  1,')  P.M.  it, 
moved  rapidly  doNN  imai'ds.  and 
in  MII  hour  or  two  depended  verl  i 
e.-illy  ,  it  Ihns  remained  all  nii-hl 
Msleep.  This  position  could  not 
he  represented  on  the  vertical 
sl.e.s  nor  in  the  lii'.nre  here  :-.i\en. 
l'-\  ('  10  V  M.  on  the  lollop  III". 

morning  (18th)    l-..th   cotyledons 

had    M  en    ;•  rent  IN  .  and    they  con 
I  miied    to    rise    tint  il  S  A  M  .  NN  hen 

Their    DION  enieiit   N\  as  t  raced  d  ur 
illp;   Ihe    \vhole  of    this    dav    Mild 
until    ihe   next     mornmiv  ;    hut    M, 
Iraciii:1.     is    not     riven,   as    it    \\a-; 
dose  IN   similar  to  Fns    I  ,">,  except 
ni"      that     the      lines     v\ere    more 
V,\W,tW.      The     colNledons    moNt-d 

7  times,  either  upwards  or  down* 

\\ard.s  ;  and  at  al>ont  I  P.M.  the 
^re.'it  iKX'turntil  sinking  move- 
men t  t'om n iciicci  I 

A  not  her   seed  I  ins,  \\  MS  t  •!  iser\  ed 
in  a  similar  manner  dnrni"  nearly 
8MO»«,«,      2lh..  hut  NNith  llu<  dillereiu-e  tha't 

the   IIN  poeotx  1    NNas   left     I'ree.         I'll.' 
movement  also  N\  as  less  m.isnilied. 
HelNNc,  n   S  \\\    \  M.  and   .'»  P.M.   on 
tin*  iSth,  the  apex    o!'   the  CO!N  le 
doll   moved   7    times    npwMl'ds    or 
etreiinuu.iii(i..n  of    downwMrds   (Fiir.  1  IX      Hie  n.v 
o..!ylo,lous,  tl>o  hv|»o,-,.j\  I  I.,  in.-      turnal   Minkin:'.    mov<  rnent.  which 
•"•"'"•li    '      •      '  ••  -  •  'll»'»'u:i-      is  merely  M   !';reMt    increase  of  , MIC 
••••I   '"""  ••<>"  ,.,„   ,],,,,„,, i   ,,•  -ill.t.on-     •  MH 

(iTcnonr  h.-iii  ofori|ift«lldil«,  lu    (linnul  OS( '" 

'it   r<iltltv{tiu<i.--  This  species  id  intcrcsiini-.  as  the  eoty 


OXA1  IS 


I.  dons  n;;r  |  >«  T|  M  -in  1  1.  MI  l:i  i'l  y   ii  |  >\\  a  r,  I  ..  :i  I   in:  lit,  RO  (IB  to  001110  into 

do:..'  rolllurl  ,   III-,  I.  -id    of   Mllklli"    v.Tt  IC!1  1  1  V   <  lo\Y  II  Wll  I'tlH,  UN   ill   lllO 
c:i  :.r    «•!'    (>.     rn.svn         \     v.l.i.:.    lihlilt'lll     \Y;i:;    li\rd     to    n,    rol  yli'dtHI, 


'17  of  Jill    nidi    in    li'iir,!  Ii,  :ind   the   In  porot \ I 
[.  U, 

20,/m  l.'l"' 


ns' 


On 


Oxnlis    I'.f/i/iV/.r/M  :     c<ni| i 

.11.  iiiiiuul.il  IUM    i»f  n  cul.ylo- 

•  loll  Mini    Mir  ll\  |mr.'l  vl,   I  II.  r.l 

"ii  \.ri»,il  ^IUMH,  (luring  24 
IK  "i  r.i.     Figure  horo  given 

<>n,    I, .ill    ,.i     ,.i  K-iniil     Mciiln  ; 

1 1  I  II 1 1 1 1 II. 1 1  ,-,  |         iV.MII 


•  .'lij.'inl  MI.  tiiniiiil.il  i. 'ii  ,.f 
(In-  (••«!  \  I.-.I.- 
I  ""•!  8.12  A.M.  on  .Inn.-  I  SI  I,  to  7. .'10 
v  M  I '.Mil.  Tli,'  uprx  ..1  Hi.-  rolylo.lol, 
nl.uoil  i.nlv  .".:;  in.  I,,  .  IV.. in  II,,-  vi<rlirnl 
(jlMM.  KJ^.III.-  her.'  -.urn  our  kill  of 

origin*]  icftli 


lirsi.  .l:iv   Mi.'   :;rr.|ln,;'    u.i      |I|II.I-<M|    loo    l.ir    Inun   Ilir 

SO   th:i|.    lh<>    InicilP          i         'h.niioii;;|y  r\:ir!-.i-r:i.lr«|  :ili. I   Ihn 
liiovrinciil.  i-iinlil   not    l,r  Ir.-i.-c.l   \\  |M>ll   III.'  ml  V  I.'.  Ion  i-ilhcr  ro::o  or 
HiiK-li;    l.ul    it.    w.'i-;  clc;irly   firrii    tluil.   Mir   r,.ly  !«•.!.  .us    rosd 
Iliri.-o  Mil. I   frll    luirr    I..  l\\,.  i,    }     If,   A  M     Illlii    I    I    •  r  AI         linlv   oil 
MIC  rollounij;  nioriiiiij',  (June  I'.MIi)  ll.r  ii|u<\  ol  u  col  vl<  .Ion  \v;i:; 


2(3  CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  L 

placed  only  l-£  inch  from  the  vertical  glass.  At  6.40  A.M.  it 
stood  horizontally ;  it  then  fell  till  8.35,  and  then  rose.  Al- 
together in  the  course  of  12  h.  it  rose  thrice  and  fell  thrice,  as 
may  be  seen  in  Fig.  15.  The  great  nocturnal  rise  of  the  coty- 
ledons usually  commences  about  4  or  5  P.M.,  and  on  the  following 
morning  they  are  expanded  or  stand  horizontally  at  about  6.30 
A.M.  In  the  present  instance,  however,  the  great  nocturnal  rise 
did  not  commence  till  7  P.M.  ;  but  this  was  due  to  the  hypocotyl 
having  from  some  unknown  cause  temporarily  bent  to  the  left 
side,  as  is  shown  in  the  tracing.  To  ascertain  positively  that 
the  hypocotyl  circumnutated,  a  mark  was  placed  at  8.15  P.M. 
behind  the  two  now  closed  and  vertical  cotyledons ;  and  the 
movement  of  a  glass  filament  fixed  upright  to  the  top  of  the 
hypocotyl  was  traced  until  10.40  P.M.  During  this  time  it 
moved  from  side  to  side,  as  well  as  backwards  and  forwards, 
plainly  showing  circumnutation ;  but  the  movement  was  small 
in  extent.  Therefore  Fig.  15  represents  fairly  well  the  move- 
ments ef  the  cotyledons  alone,  with  the  exception  of  the  one 
great  afternoon  curvature  to  the  left. 

Oxalis  cornicidata  (var.  cuprea). — The  cotyledons  rise  at  night 
to  a  variable  degree  above  the  horizon,  generally  about  45° : 
those  on  some  seedlings  between  2  and  5  days  old  were  found 
to  be  in  continued  movement  all  day  long ;  but  the  movements 
were  more  simple  than  in  the  last  two  species.  This  may  have 
partly  resulted  from  their  not  being  sufficiently  illuminated 
whilst  being  observed,  as  was  shown  by  their  not  beginning  to 
rise  until  very  late  in  the  evening. 

Oxalis  (Biopliytwm)  sensitiva. — The  cotyledons  are  highly  re- 
markable from  the  amplitude  and  rapidity  of  their  movements 
during  the  day.  The  angles  at  which  they  stood  above  or 
beneath  the  horizon  were  measured  at  short  intervals  of  time ; 
and  we  regret  that  their  course  was  not  traced  during  the  whole 
day.  "We  will  give  only  a  few  of  the  measurements,  which  were 
made  whilst  the  seedlings  were  exposed  to  a  temperature  of  22  2° 
to  24s0  C.  One  cotyledon  rose  70°  in  11  m. ;  another,  on  a  distinct 
seedling,  fell  80°  in  12m.  Immediately  before  this  latter  fall 
the  same  cotyledon  had  risen  from  a  vertically  downward  to  a 
vertically  upward  position  in  1  h.  48  m.,  and  had  therefore  passed 
through  180°  in  under  2  h.  We  have  met  with  no  other  instance 
of  a  circumnutating  movement  of  such  great  amplitude  as  180° ; 
nor  of  such  rapidity  of  movement  as  the  passage  through  80°  in 
12m.  The  cotyledons  of  this  plant  sleep  at  night  by  rising 


CHAP.  I. 


TROP^EOLUM. 


27 


Fig.  16. 


vertically  and  coming  into  close  contact.  This  upward  move- 
ment differs  from  one  of  the  great  diurnal  oscillations  above 
described  only  by  the  position  being  permanent  during  the  night 
and  by  its  periodicity,  as  it  always  commences  late  in  the 
evening. 

Tropxolum  minus  (?)  (var.  Tom  Thumb)  (Tropseoleae). — The 
cotyledons  are  hypogean,  or  never  rise  above  the  ground.  By 
removing  the  soil  a  buried  epicotyl 
or  plumule  was  found,  with  its 
summit  arched  abruptly  down- 
wards, like  the  arched  hypocotyl 
of  the  cabbage  previously  described. 
A  glass  filament  with  a  bead  at 
its  end  was  affixed  to  the  basal  half 
or  leg,  just  above  the  hypogean 
cotyledons,  which  were  again  almost 
surrounded  by  loose  earth.  The 
tracing  (Fig.  16)  shows  the  course 
of  the  bead  during  11  h.  After  the 
last  dot  given  in  the  figure,  the 
bead  moved  to  a  great  distance, 
and  finally  off  the  glass,  in  the 
direction  indicated  by  the  broken 
line.  This  great  movement,  due  to 
increased  growth  along  the  con- 
cave surface  of  the  arch,  was  caused 
by  the  basal  leg  bending  back- 
wards from  the  upper  part,  that  is 
in  a  direction  opposite  to  the  depen- 
dent tip,  in  the  same  manner  as 
occurred  with  the  hypocotyl  of 
the  cabbage.  Another  buried  and 

arched  epicotyl  was  observed  in  the  same  manner,  excepting 
that  the  two  legs  of  the  arch  were  tied  together  with  fine  silk 
for  the  sake  of  preventing  the  great  movement  just  mentioned. 
It  moved,  however,  in  the  evening  in  the  same  direction  as 
before,  but  the  line  followed  was  not  so  straight.  During  the 
morning  the  tied  arch  moved  in  an  irregularly  circular,  strongly 
zigzag  course,  and  to  a  greater  distance  than  in  the  previous 
case,  as  was  shown  in  a  tracing,  magnified  18  times.  The  move- 
ments of  a  young  plant  bearing  a  few  leaves  and  of  a  mature 
plant,  will  hereafter  be  described. 


Tropceolum  minus  (?) :  circum- 
nutation  of  buried  and  arched 
epicotyl,  traced  on  a  horizon- 
tal glass,  from  9.20  A.M.  to 
8.15  P.M.  Movement  of  bead 
of  filament  magnified  27 
times. 


28  CIRCUMNUTATION  OF  SEEDLINGS.         CHAP.  I. 

Citrus  aurantium  (Orange)  (Aurantiaceaj). — The  cotyledons 
are  hypogean.  The  circumnutation  of  an  epicotyl,  which  at  the 
close  of  our  observations  was  '59  of  an  inch  (15  mm.)  in  height 
above  the  ground,  is  shown  in  the  annexed  figure  (Fig.  17),  as 
observed  during  a  period  of  44  h.  40  m. 

Fig.  17. 


Citrus  aurantium :  circumnutation  of  epicotyl  with  a  filament  fixed  trans- 
versely near  its  apex,  traced  on  a  horizontal  glass,  from  12.13  P.M.  on 
Feb.  20th  to  8.55  A.M.  on  22nd.  The  movement  of  the  bead  of  the 
filament  was  at  first  magnified  21  times,  or  10^,  in  figure  here  given, 
and  afterwards  36  times,  or  18  as  here  given ;  seedling  illuminated 
from  above. 

jfisculus  liippocastanum  (Hippocastaneae). — Germinating  seeds 
were  placed  in  a  tin  box,  kept  moist  internally,  with  a  sloping 
bank  of  damp  argillaceous  sand,  on  which  four  smoked  glass- 
plates  rested,  inclined  at  angles  of  70°  and  65°  with  the 
horizon.  The  tips  of  the  radicles  were  placed  so  as  just  to 
touch  the  upper  end  of  the  glass-plates,  and,  as  they  grew 
downwards  they  pressed  lightly,  owing  to  geotropism,  on  the 
smoked  surfaces,  and  left  tracks  of  their  course.  In  the  middle 
part  of  each  track  the  glass  was  swept  clean,  but  the  margins 
were  much  blurred  and  irregular.  Copies  of  two  of  these  tracks 
(all  four  being  nearly  alike)  were  made  on  tracing  paper  placed 
over  the  glass-plates  after  they  had  been  varnished ;  and  they 
are  as  exact  as  possible,  considering  the  nature  of  the  margins 
(Fig.  18).  They  suffice  to  show  that  there  was  some  lateral, 
almost  serpentine  movement,  and  that  the  tips  in  their  down- 
ward course  pressed  with  unequal  force  on  the  plates,  as 


CHAP.  I. 


VICIA. 


29 


Fig.  18. 


the  tracks  varied  in  breadth.  The  more  perfectly  serpentine 
tracks  made  by  the  radicles  of  Phaseolus  multiflorus  and  Vicia 
faba  (presently  to  be  described),  render 
it  almost  certain  that  the  radicles  of 
the  present  plant  circumnutated. 

Phaseolus  multiftorus  (LeguminosaD). 
—Four  smoked  glass-plates  were  ar- 
ranged in  the  same  manner  as  des- 
cribed under  JEsculus,  and  the  tracks 
left  by  the  tips  of  four  radicles  of  the 
present  plant,  whilst  growing  down- 
wards, were  photographed  as  trans- 
parent objects. 

here  exactly  copied  (Fig.  19).  Their 
serpentine  courses  show  that  the  tips 
moved  regularly  from  side  to  side; 
they  also  pressed  alternately  with 
greater  or  less  force  on  the  plates, 
sometimes  rising  up  and  leaving  them 
altogether  for  a  very  short  distance ; 
but  this  was  better  seen  on  the 
original  plates  than  in  the  copies. 
These  radicles  therefore  were  continually  moving  in  all  direc- 
tions— that  is,  they  circumnutated.  The  distance  between  the 
extreme  right  and  left  positions 
of  the  radicle  A,  in  its  lateral 
movement,  was  2  mm.,  as  ascer- 
tained by  measurement  with  an 
eye-piece  micrometer. 

Vicia  faba  (Common  Bean) 
(Leguminosss). —  fiadicle.  — Some 
beans  were  allowed  to  germinate 
on  bare  sand,  and  after  one  had 
protruded  its  radicle  to  a  length 
of  "2  of  an  inch,  it  was  turned 
upside  down,  so  that  the  radicle, 
which  was  kept  in  damp  air, 
now  stood  upright.  A  filament, 
nearly  an  inch  in  length,  was 
affixed  obliquely  near  its  tip;  and  the  movement  of  the 
terminal  bead  was  traced  from  8.30  A.M.  to  10.30  P.M.,  as  shown 
in  Fig.  18.  The  radicle  at  first  changed  its  course  twice 


A. 

Three  of  them  are  ^scalw  hippocastanum :  out- 
lines of  tracks  left  on  in- 
clined glass-plates  by  tips 
of  radicles.  In  A  the  plate 
was  inclined  at  70°  with 
the  horizon,  and  the  radicle 
was  1  •  9  inch  in  length,  and 
•23  inch  in  diameter  at  base. 
In  B  the  plate  was  inclined 
65°  with  the  horizon,  and 
the  radicle  was  a  trifle 
larger. 


Fig.  19. 


B.  C. 

Phaseolus  multiflorus :  tracks  left 
on  inclined  smoked  glass-plates 
by  tips  of  radicles  in  growing 
downwards.  A  and  C,  plates 
inclined  at  60°,  B  inclined  at 
68°  with  the  horizon. 


30 


CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 


abruptly,  then  made  a  small  loop  and  then  a  larger  zigzag 
curve.      During  the  night  and  till  11  A.M.  on  the  following 

Fig.  20. 


Vicia  faba :  circumnutation  of  a  radicle,  at  first  pointing  vertically  up- 
wards, kept  in  darkness,  traced  on  a  horizontal  glass,  during  14  hours. 
Movement  of  bead  of  filament  magnified  23  times,  here  reduced  to 
one-half  of  original  scale. 

morning,  the  bead  moved  to  a  great  distance  in  a  nearly  straight 
line,  in  the  direction  indicated  by  the  broken  line  in  the  figure. 
This  resulted  from  the  tip  bending  quickly  downwards,  as  it 
had  now  become  much  declined,  and  had  thus  gained  a  position 
highly  favourable  for  the  action  of  geotropism. 

Fig.  21. 


D. 


E. 


Ttcia  faba :  tracks  left  on  inclined  smoked  glass-plates,  by  tips  of  radicles 
in  growing  downwards.  Plate  C  was  inclined  at  63°,  plates  A  and  D 
at  71°,  plate  B  at  75°,  and  plate  E  at  a  few  degrees  beneath  the 
horizon. 


CHAP.  I.  VICIA.  31 

We  next  experimented  on  nearly  a  score  of  radicles  by  allowing 
them  to  grow  downwards  over  inclined  plates  of  smoked  glass, 
in  exactly  the  same  manner  as  with  JEsculus  and  Phaseolus. 
Some  of  the  plates  were  inclined  only  a  few  degrees  beneath 
the  horizon,  but  most  of  them  between  60°  and  75°.  In  the 
latter  cases  the  radicles  in  growing  downwards  were  deflected 
only  a  little  from  the  direction  which  they  had  followed  whilst 
germinating  in  sawdust,  and  they  pressed  lightly  on  the  glass- 
plates  (Fig.  21).  Five  of  the  most  distinct  tracks  are  here 
copied,  and  they  are  all  slightly  sinuous,  showing  circumnuta- 
tion.  Moreover,  a  close  examination  of  almost  every  one  of  the 
tracks  clearly  showed  that  the  tips  in  their  downward  course 
had  alternately  pressed  with  greater  or  less  force  on  the  plates, 
and  had  sometimes  risen  up  so  as  nearly  to  leave  them  for  short 
intervals.  The  distance  between  the  extreme  right  and  left 
positions  of  the  radicle  A  was  0'7  mm.,  ascertained  in  the  same 
manner  as  in  the  case  of  Phaseolus. 

Epicotyl. — At  the  point  where  the  radicle  had  protruded  from 
a  bean  laid  on  its  side,  a  flattened  solid  lump  projected  "1  of  an 
inch,  in  the  same  horizontal  plane  with  the  bean.  This  protuber- 
ance consisted  of  the  convex  summit  of  the  arched  epicotyl; 
and  as  it  became  developed  the  two  legs  of  the  arch  curved 
themselves  laterally  upwards,  owing  to  apogeotropism,  at  such 
a  rate  that  the  arch  stood  highly  inclined  after  14  h.,  and 
vertically  in  48  h.  A  filament  was  fixed  to  the  crown  of 
the  protuberance  before  any  arch  was  visible,  but  the  basal 
half  grew  so  quickly  that  on  the  second  morning  the  end  of  the 
filament  was  bowed  greatly  downwards.  It  was  therefore  re- 
moved and  fixed  lower  down.  The  line  traced  during  these  two 
days  extended  in  the  same  general  direction,  and  was  in  parts 
nearly  straight,  and  in  others  plainly  zigzag,  thus  giving  some 
evidence  of  circumnutation. 

As  the  arched  epicotyl,  in  whatever  position  it  may  be  placed, 
bends  quickly  upwards  through  apogeotropism,  and  as  the  two 
legs  tend  at  a  very  early  age  to  separate  from  one  another,  as 
soon  as  they  are  relieved  from  the  pressure  of  the  surrounding 
earth,  it  was  difficult  to  ascertain  positively  whether  the  epicotyl, 
whilst  remaining  arched,  circumnutated.  Therefore  some  rather 
deeply  buried  beans  were  uncovered,  and  the  two  legs  of  the 
arches  were  tied  together,  as  had  been  done  with  the  epicotyl 
of  Tropseolum  and  the  hypocotyl  of  the  Cabbage.  The  move- 
ments of  the  tied  arches  were  traced  in  the  usual  manner  on 


32  CIRCUMNUTATION   OF  SEEDLINGS.          CHAP.  I. 

two  occasions  during  three  days.  But  the  tracings  made  under 
sach  unnatural  conditions  are  not  worth  giving ;  and  it  need 
only  be  said  that  the  lines  were  decidedly  zigzag,  and  that 
small  loops  were  occasionally  formed.  We  may  therefore  con- 
clude that  the  epicotyl  circumnutates  whilst  still  arched  and 
before  it  has  grown  tall  enough  to  break  through  the  surface 
of  the  ground. 

In  order  to  observe  the  movements  of  the  epicotyl  at  a  some- 
what more  advanced  age,  a  filament  was  fixed  near  the  base  of 
one  which  was  no  longer  arched,  for  its  upper  half  now  formed 
a  right  angle  with  the  lower  half.  This  bean  had  germinated 
on  bare  damp  sand,  and  the  epicotyl  began  to  straighten  itself 
much  sooner  than  would  have  occurred  if  it  had  been  properly 
planted.  The  course  pursued  during  50  h.  (from  9  A.M.  Dec. 
26th,  to  11  A.M.  28th)  is  here  shown  (Fig.  22) ;  and  we  see 

Fig.  22. 


Vida  faba :  circumnutation  of  young  epicotyl,  traced  in  darkness  during 
50  hours  on  a  horizontal  glass.  Movement  of  bead  of  filament  mag- 
nified 20  times,  here  reduced  to  one-half  of  original  scale. 

that  the  epicotyl  circumnutated  during  the  whole  time.  Its 
basal  part  grew  so  much  during  the  50  h.  that  the  filament 
at  the  end  of  our  observations  was  attached  at  the  height  of 
•4  inch  above  the  upper  surface  of  the  bean,  instead  of  close 
to  it.  If  the  bean  had  been  properly  planted,  this  part  of  the 
epicotyl  would  still  have  been  beneath  the  soil. 

Late  in  the  evening  of  the  28th,  some  hours  after  the  above 
observations  were  completed,  the  epicotyl  had  grown  much 
straighter,  for  the  upper  part  now  formed  a  widely  open  angle 
with  the  lower  part.  A  filament  was  fixed  to  the  upright  basal 
part,  higher  up  than  before,  close  beneath  the  lowest  scale-like 
process  or  homologue  of  a  leaf;  and  its  movement  was  traced 


CHAP.  I. 


LA.THYKUS. 


33 


during  38  h.  (Fig.  23).  We  here  again  have  plain  evidence  of 
continued  circumnutation.  Had  the  bean  been  properly  planted, 
the  part  of  the  epicotyl  to  which  the  filament  was  attached,  tho 

Fig.  23. 


Viciafaba:  circumnutation  of  the  same  epicotyl  as  in  Fig.  22,  a  little  more 
advanced  in  age,  traced  under  similar  conditions  as  before,  from  8.40  A.M. 
Dec.  28th,  to  10.50  A.M.  30th.  Movement  of  bead  here  magnified 
20  times. 

movement  of  which  is  here  shown,  would  probably  have  just 
risen  above  the  surface  of  the  ground. 

Lathyrus  nissolia  (Leguminosse). — This  plant  was  selected  for 
observation  from  being  an  abnormal  form  with  grass-like  leaves. 

Fig.  24. 


Lathyrus  nissolia:  circumnutation  of  stem  of  young  seedling,  traced  in 
darkness  on  a  horizontal  glass,  from  6.45  A.M.  Nov.  22nd,  to  7  A.M. 
23rd.  Movement  of  end  of  leaf  magnified  about  12  times,  here  re- 
duced to  one-half  of  original  scale. 

The  cotyledons  are  hypogean,  and  the  epicotyl  breaks  through 
the  ground  in  an  arched  form.  The  movements  of  a  stem,  1/2 
inch  in  height,  consisting  of  three  internodes,  the  lower  one 
almost  wholly  subterranean,  and  the  upper  one  bearing  a  short, 


34  CIKCUMNUTAT1ON   OF  SEEDLINGS.         CHAP.  I. 

narrow  leaf,  is  shown  during  24  h.,  in  Fig.  24.  No  glass  filament 
was  employed,  but  a  mark  was  placed  beneath  the  apex  of  the 
leaf.  The  actual  length  of  the  longer  of  the  two  ellipses  de- 
scribed by  the  stem  was  about  •  14  of  an  inch.  On  the  previous 
day  the  chief  line  of  movement  was  nearly  at  right  angles  to 
that  shown  in  the  present  figure,  and  it  was  more  simple. 

Cassia,  tora*  (Leguminosse). — A  seedling  was  placed  before  a 


Cassia  tora :  conjoint  circumnutation  of  cotyledons  and  hypocotyl,  traced 
on  vertical  glass,  from  7.10  A.M.  Sept.  25th  to  7.30  A.M.  26th.  Figure 
here  given  reduced  to  one-half  of  original  scale. 


*  Seeds  of  tl.is   plant,   which  flourish  or  flower  well  with  us; 

grew  near  the  sea-sido,  were  sent  they  were  sent  to  Kew,  and  were 

to  us   by  Fritz  Miiller  from  S.  pronounced  not  to  be  distinguish- 

Erazil.      The  seedlings  did  not  able  from  C.  tora. 


CHAP.  I.  LOTUS.  35 

north-east  window ;  it  bent  very  little  towards  it,  as  the  hypo- 
cotyl  which  was  left  free  was  rather  old,  and  therefore  not  highly 
heliotropic.  A  filament  had  been  fixed  to  the  midrib  of  one  of 
the  cotyledons,  and  the  movement  of  the  whole  seedling  was 
traced  during  two  days.  The  circumnutation  of  the  hypocotyl 
is  quite  insignificant  compared  with  that  of  the  cotyledons. 
These  rise  up  vertically  at  night  and  come  into  close  contact ;  so 
that  they  may  be  said  to  sleep.  This  seedling  was  so  old  that  a 
very  small  true  leaf  had  been  developed,  which  at  night  was 
completely  hidden  by  the  closed  cotyledons.  On  Sept.  24th, 
between  8  A.M.  and  5  P.M.,  the  cotyledons  moved  five  times  up 
and  five  times  down;  they  therefore  described  five  irregular 
ellipses  in  the  course  of  the  9  h.  The  great  nocturnal  rise  com- 
menced about  4.30  P.M. 

On  the  following  morning  (Sept.  25th)  the  movement  of 
the  same  cotyledon  was  again  traced  in  the  same  manner 
during  24  h. ;  and  a  copy  of  the  tracing  is  here  given  (Fig.  25). 
The  morning  was  cold,  and  the  window  had  been  accidentally 
left  open  for  a  short  time,  which  must  have  chilled  the  plant ; 
and  this  probably  prevented  it  from  moving  quite  as  freely  as 
on  the  previous  day ;  for  it  rose  only  four  and  sank  only  four 
times  during  the  day,  one  of  the  oscillations  being  very  small. 
At  7.10  A.M.,  when  the  first  dot  was  made,  the  cotyledons  were 
not  fully  open  or  awake ;  they  continued  to  open  till  about  9  A.M., 
by  which  time  they  had  sunk  a  little  beneath  the  horizon :  by 
9.30  A.M.  they  had  risen,  and  then  they  oscillated  up  and  down  ; 
but  the  upward  and  downward  lines  never  quite  coincided.  At 
about  4.30  P.M.  the  great  nocturnal  rise  commenced.  At  7  A.M. 
on  the  following  morning  (Sept.  26th)  they  occupied  nearly 
the  same  level  as  on  the  previous  morning,  as  shown  in  the 
diagram :  they  then  began  to  open  or  sink  in  the  usual  manner. 
The  diagram  leads  to  the  belief  that  the  great  periodical  daily 
rise  and  fall  does  not  differ  essentially,  excepting  in  amplitude, 
from  the  oscillations  during  the  middle  of  the  day. 

Lotus  Jacobceus  (Leguminosse). — The  cotyledons  of  this  plant, 
after  the  few  first  days  of  their  life,  rise  so  as  to  stand  almost, 
though  rarely  quite,  vertically  at  night.  They  continue  to  act  in 
this  manner  for  a  long  time  even  after  the  development  of  some 
of  the  true  leaves.  With  seedlings,  3  inches  in  height,  and  bear- 
ing five  or  six  leaves,  they  rose  at  night  about  45°.  They  con- 
tinued to  act  thus  for  about  an  additional  fortnight.  Subse- 
quently they  remained  horizontal  at  night,  though  still  green, 


36  CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 

and  at  last  dropped  off.  Their  rising  at  night  so  as  to  stand 
almost  vertically  appears  to  depend  largely  on  temperature ; 
for  when  the  seedlings  were  kept  in  a  cool  house,  though  they 
still  continued  to  grow,  the  cotyledons  did  not  become  vertical 
at  night.  It  is  remarkable  that  the  cotyledons  do  not  generally 
rise  at  night  to  any  conspicuous  extent  during  the  first  four  or 
five  days  after  germination;  but  the  period  was  extremely 
variable  with  seedlings  kept  under  the  same  conditions;  and 
many  were  observed.  Glass  filaments  with  minute  triangles  of 
paper  were  fixed  to  the  cotyledons  (Is  mm.  in  breadth)  of  two 
seedlings,  only  24  h.  old,  and  the  hypocotyl  was  secured  to  a 
stick ;  their  movements  greatly  magnified  were  traced,  and  they 
certainly  circumnutated  the  whole  time  on  a  small  scale,  but 
they  did  not  exhibit  any  distinct  nocturnal  and  diurnal  move- 
ment. The  hypocotyls,  when  left  free,  circumnutated  over  a 
large  space. 

Another  and  much  older  seedling,  bearing  a  half-developed 
leaf,  had  its  movements  traced  in  a  similar  manner  during  the 
three  first  days  and  nights  of  June ;  but  seedlings  at  this  age 
appear  to  be  very  sensitive  to  a  deficiency  of  light ;  they  were 
observed  under  a  rather  dim  skylight,  at  a  temperature  of 
be-tween  16°  to  17£°  C. ;  and  apparently,  in  consequence  of  these 
conditions,  the  great  daily  movement  of  the  cotyledons  ceased 
on  the  third  day.  During  the  first  two  days  they  began  rising 
in  the  early  afternoon  in  a  nearly  straight  line,  until  between 
6  and  7  P.M.,  when  they  stood  vertically.  During  the  latter 
part  of  the  night,  or  more  probably  in  the  early  morning,  they 
began  to  fall  or  open,  so  that  by  6.45  A.M.  they  stood  fully 
expanded  and  horizontal.  They  continued  to  fall  slowly  for 
some  time,  and  during  the  second  day  described  a  single 
small  ellipse,  between  9  A.M.  and  2  P.M.,  in  addition  to  the 
great  diurnal  movement.  The  course  pursued  during  the 
whole  24  h.  was  far  less  complex  than  in  the  foregoing  case  of 
Cassia.  On  the  third  morning  they  fell  very  much,  and  then 
circumnutated  on  a  small  scale  round  the  same  spot ;  by  8.20 
P.M.  they  showed  no  tendency  to  rise  at  night.  Nor  did  the 
cotyledons  of  any  of  the  many  other  seedlings  in  the  same  pot 
rise ;  and  so  it  was  on  the  following  night  of  June  5th.  The 
pot  was  then  taken  back  into  the  hot-house,  where  it  was  exposed 
to  the  sun,  and  on  the  succeeding  night  all  the  cotyledons  rose 
again  to  a  high  angle,  but  did  not  stand  quite  vertically.  On 
each  of  the  above  days  the  line  representing  the  great  nocturnal 


CHAP.  I.  CYTISUS.  37 

rise  did  not  coincide  with  that  of  the  great  diurnal  fall,  so  that 
narrow  ellipses  were  described,  as  is  the  usual  rule  with  circum- 
nutating  organs.  The  cotyledons  are  provided  with  a  pulvinus, 
and  its  development  will  hereafter  be  described. 

Mimosa  pudica  (Leguminosse). — The  cotyledons  rise  up  verti- 
cally at  night,  so  as  to  close  together.  Two  seedlings  were 
observed  in  the  greenhouse  (temp.  16°  to  17°  C.  or  63°  to  65°  F.). 
Their  hypocotyls  were  secured  to  sticks,  and  glass  filaments 
bearing  little  triangles  of  paper  were  affixed  to  the  cotyledons  of 
both.  Their  movements  were  traced  on  a  vertical  glass  during 
24  h.  on  November  13th.  The  pot  had  stood  for  some  time  in 
the  same  position,  and  they  were  chiefly  illuminated  through 
the  glass-roof.  The  cotyledons  of  one  of  these  seedlings  moved 
downward  in  the  morning  till  11.30  A.M.,  and  then  rose,  moving 
rapidly  in  the  evening  until  they  stood  vertically,  so  that  in  this 
case  there  was  simply  a  single  great  daily  fall  and  rise.  The 
other  seedling  behaved  rather  differently,  for  it  fell  in  the  morn- 
ing until  11.30  A.M.,  and  then  rose,  but  after  12.10  P.M.  again  fell ; 
and  the  great  evening  rise  did  not  begin  until  1.22  P.M.  On  the 
following  morning  this  cotyledon  had  fallen  greatly  from  its 
vertical  position  by  8.15  A.M.  Two  other  seedlings  (one  seven 
and  the  other  eight  days  old)  had  been  previously  observed 
under  unfavourable  circumstances,  for  they  had  been  brought 
into  a  room  and  placed  before  a  north-east  window,  where  the 
temperature  was  between  only  56°  and  57°  F.  They  had,  more- 
over, to  be  protected  from  lateral  light,  and  perhaps  were  not 
sufficiently  illuminated.  Under  these  circumstances  the  coty- 
ledons moved  simply  downwards  from  7  A.M.  till  2  P.M.,  after 
which  hour  and  during  a  large  part  of  the  night  they  con- 
tinued to  rise.  Between  7  and  8  A.M.  on  the  following  morning 
they  fell  again ;  but  on  this  second  and  likewise  on  the  third 
day  the  movements  became  irregular,  and  between  3  and  10.30 
P.M.  they  circumnutated  to  a  small  extent  about  the  same  spot ; 
but  they  did  not  rise  at  night.  Nevertheless,  on  the  following 
night  they  rose  as  usual. 

Cytisus  fragrans  (Leguminosae). — Only  a  few  observations  were 
made  on  this  plant.  The  hypocotyl  circumnutated  to  a  con- 
siderable extent,  but  in  a  simple  manner — namely,  for  two  hours 
in  one  direction,  and  then  much  more  slowly  back  again  in 
a  zigzag  course,  almost  parallel  to  the  first  line,  and  beyond  the 
starting-point.  It  moved  in  the  same  direction  all  night,  but 
next  morning  began  to  return.  The  cotyledons  continually 


38  cracuMNUTA'noN  OF  SEEDLINGS.      CHAP.  T. 

move  both  up  and  down  and  laterally ;  but  they  do  not  rise  up 
at  night  in  a  conspicuous  manner. 

Lupinus  luteus  (Leguminosre). — Seedlings  of  this  plant  were 
observed  because  the  cotyledons  are  so  thick  (about  '03  of  an 
inch)  that  it  seemed  unlikely  that  they  would  move.  Our 
observations  were  not  very  successful,  as  the  seedlings  are 
strongly  heliotropic,  and  their  circumnutation  could  not  be 
accurately  observed  near  a  north-east  window,  although  they 
had  been  kept  during  the  previous  day  in  the  same  position. 
A  seedling  was  then  placed  in  darkness  with  the  hypocotyl 
secured  to  a  stick;  both  cotyledons  rose  a  little  at  first,  and 
then  fell  during  the  rest  of  the  day ;  in  the  evening  between 
5  and  6  P.M.  they  moved  very  slowly ;  during  the  night  one 
continued  to  fall  and  the  other  rose,  though  only  a  little.  The 
tracing  was  not  much  magnified,  and  as  the  lines  were  plainly 
zigzag,  the  cotyledons  must  have  moved  a  little  laterally,  that 
is,  they  must  have  circumnutated. 

The  hypocotyl  is  rather  thick,  about  •  12  of  inch ;  nevertheless 
it  circumnutated  in  a  complex  course,  though  to  a  small  extent. 
The  movement  of  an  old  seedling  with  two  true  leaves  partially 
developed,  was  observed  in  the  dark.  As  the  movement  was 
magnified  about  100  times  it  is  not  trustworthy  and  is  not 
given ;  but  there  could  be  no  doubt  that  the  hypocotyl  moved 
in  all  directions  during  the  day,  changing  its  course  19  times. 
The  extreme  actual  distance  from  side  to  side  through  which 
the  upper  part  of  the  hypocotyl  passed  in  the  course  of  14:5  hours 
was  only  ^V  of  an  incn  5  it  sometimes  travelled  at  the  rate  of 
•gL.  of  an  inch  in  an  hour. 

Cucurbita  ovifera  (Cucurbitacece). —  Radicle  :  a  seed  which  had 

Fig.  26. 


Cucurbita  ov>f era:  course  followed  by  a  radicle  in  bending  geotropically 
downwards,  traced  on  a  horizontal  glass,  between  11.25  A.M.  and  10.25 
P.M.  ;  the  direction  during  the  night  is  indicated  by  the  broken  line. 
Movement  of  bead  magnified  14  times. 

germinated  on  damp  sand  was  fixed  so  that  the  slightly  curved 
radicle,  which  was  only  '07  inch  in  length,  stood  almost  vertically 


CHAP.  I. 


CUCURBITA. 


39 


upwards,  in  which  position  geotropism  would  act  at  first  with 
little  power.  A  filament  was  attached  near  to  its  base,  and 
projected  at  about  an  angle  of  45°  above  the  horizon.  The 
general  course  followed  during  the  11  hours  of  observation  and 
during  the  following  night,  is  shown  in  the  accompanying 
diagram  (Fig.  26),  and  was  plainly  due  to  geotropism ;  but  it 
was  also  clear  that  the  radicle  circumnutated.  By  the  next 
morning  the  tip  had  curved  so  much  downwards  that  the  fila- 
ment, instead  of  projecting  at  45°  above  the  horizon,  was  nearly 
horizontal.  Another  germinating  seed  was  turned  upside  down 
and  covered  with  damp  sand ;  and  a  filament  was  fastened  to 
the  radicle  so  as  to  project  at  an  angle  of  about  50°  above  the 
horizon;  this  radicle  was  '35  of  an  inch  in  length  and  a  little 
curved.  The  course  pursued  was  mainly  governed,  as  in  the 
last  case,  by  geotropism,  but  the  line  traced  during  12  hours  and 
magnified  as  before  was  more  strongly  zigzag,  again  showing 
circumnutation. 

Four  radicles  were  allowed  to  grow  downwards  over  plates 
of  smoked  glass,  inclined  at  70°  to  the  horizon,  under  the 


Fig.  27. 


A.  B. 

Cucurbita  ovifera :  tracks 
left  by  tips  of  radicles 
in  growing  downwards 
over  smoked  glass- 
plates,  inclined  at  70° 
to  the  horizon. 


Cucurbita  ovifera :  circumnuta- 
tion of  arched  hypocotyl  at 
a  very  early  age,  traced  in 
darkness  on  a  horizontal  glass, 
from  8  A.M.  to  10.20  A.M.  on 
the  following  day.  The  move- 
ment of  the  bead  magnified 
20  times,  here  reduced  to  one- 
half  of  original  scale. 


same  conditions  as  in  the  cases  of  JEsculus,  Phaseolus,  and 
Vicia.  Facsimiles  are  here  given  (Fig.  27)  of  two  of  these 
track's ;  and  a  third  short  one  was  almost  as  plainly  serpentine 
as  that  at  A.  It  was  also  manifest  by  a  greater  or  less  amount 
of  soot  having  been  swept  off  the  glasses,  that  the  tips  had 
3 


40 


CIKCUMNUTATION  OF   SEEDLINGS.         CHAP.  I. 


Fig.  29. 


pressed  alternately  with  greater  and  less  force  on  them.  There 
must,  therefore,  have  been  movement  in  at  least  two  planes  at 
right  angles  to  one  another.  These  radicles  were  so  delicate  that 
they  rarely  had  the  power  to  sweep  the  glasses  quite  clean.  One 
of  them  had  developed  some  lateral  or  secondary  rootlets,  which 
projected  a  few  degrees  beneath  the  horizon ;  and  it  is  an  im- 
portant fact  that  three  of  them  left  distinctly  serpentine  tracks 
on  the  smoked  surface,  showing  beyond  doubt  that  they  had 
circumnutated  like  the  main  or  primary  radicle.  But  the 
tracks  were  so  slight  that  they  could  not  be  traced  and  copied 
after  the  smoked  surface  had  been  varnished. 

Hypocotyl. — A  seed  lying  on  damp  sand  was  firmly  fixed  by 
two  crossed  wires  and  by  its  own  growing  radicle.  The  cotyle- 
dons were  still  enclosed  within  the  seed-coats;  and  the  short 

hypocotyl,  between  the  summit  of 
the  radicle  and  the  cotyledons, 
was  as  yet  only  slightly  arched.  A 
filament  ('85  of  inch  in  length) 
was  attached  at  an  angle  of  35° 
above  the  horizon  to  the  side  of 
the  arch  adjoining  the  cotyle- 
dons. This  part  would  ultimately 
form  the  upper  end  of  the  hypo- 
cotyl, after  it  had  grown  straight 
and  vertical.  Had  the  seed  been 
properly  planted,  the  hypocotyl  at 
this  stage  of  growth  would  have 
been  deeply  buried  beneath  the 
surface.  The  course  followed  by 
the  bead  of  the  filament  is  shown 
in  Fig.  28.  The  chief  lines  of 

fastened   transversely  across    movement  from  left  to  right  in  the 
its  upper  end,  traced  in  dark-    figlire  wero  parallel    to  the   plane 

from  Tat?  A^^tr^SO81?  M'  of  the  two  united  cotyledons  and 
The  movement  of  the  terminal  of  the  flattened  seed;  and  this 
bead  originally  magnified  movement  would  aid  in  dragging 
them  out  of  the  seed-coats,  which 
are  held  down  by  a  special  struc- 
ture hereafter  to  be  described.  The  movement  at  right  angles 
to  the  above  lines  was  due  to  the  arched  hypocotyl  becoming 
more  arched  as  it  increased  in  height.  The  foregoing  observa- 
tions apply  to  the  leg  of  the  arch  next  to  the  cotyledons,  but 


Cucurbita  ovifera:  circumnuta- 
tion  of  straight  and  verti- 
cal hypocotyl,  with  filament 


about  18  times,  here  only  4± 


CHAP.  I.  CUCUEBITA.  41 

the  other  leg  adjoining  the  radicle  likewise  circumnutated  at  an 
equally  early  age. 

The  movement  of  the  same  hypocotyl  after  it  had  become 
straight  and  vertical,  but  with  the  cotyledons  only  partially 
expanded,  is  shown  in  Fig.  29.  The  course  pursued  during  12  h. 
apparently  represents  four  and  a  half  ellipses  or  ovals,  with 
the  longer  axis  of  the  first  at  nearly  right  angles  to  that  of  the 
others.  The  longer  axes  of  all  were  oblique  to  a  line  joining 
the  opposite  cotyledons.  The  actual  extreme  distance  from 
side  to  side  over  which  the  summit  of  the  tall  hypocotyl 
passed  in  the  course  of  12  h.  was  •  28  of  an  inch.  The  original 
figure  was  traced  on  a  large  scale,  and  from  the  obliquity  of 
the  line  of  view  the  outer  parts  of  the  diagram  are  much 
exaggerated. 

Cotyledons. — On  two  occasions  the  movements  of  the  cotyle- 
dons were  traced  on  a  vertical  glass,  and  as  the  ascending  and 
descending  lines  did  not  quite  coincide,  very  narrow  ellipses 
were  formed;  they  therefore  circumnutated.  Whilst  young 
they  rise  vertically  up  at  night,  but  their  tips  always  remain 
reflexed ;  on  the  following  morning  they  sink  down  again.  With 
a  seedling  kept  in  complete  darkness  they  moved  in  the  same 
manner,  for  they  sank  from  8.45  A.M.  to  4.30P.M.;  they  then 
began  to  rise  and  remained  close  together  until  10  P.M.,  when 
they  were  last  observed.  At  7  A.M.  on  the  following  morning 
they  were  as  much  expanded  as  at  any  hour  on  the  previous 
day.  The  cotyledons  of  another  young  seedling,  exposed  to  the 
light,  were  fully  open  for  the  first  time  on  a  certain  day,  but 
were  found  completely  closed  at  7  A.M.  on  the  following  morning. 
They  soon  began  to  expand  again,  and  continued  doing  so  till 
about  5  P.M.  ;  they  then  began  to  rise,  and  by  10.30  P.M.  stood 
vertically  and  were  almost  closed.  At  7  A.M.  on  the  third  morn- 
ing they  were  nearly  vertical,  and  again  expanded  during  the 
day;  on  the  fourth  morning  they  were  not  closed,  yet  they 
opened  a  little  in  the  course  of  the  day  and  rose  a  little  on  the 
following  night.  By  this  time  a  minute  true  leaf  had  become 
developed.  Another  seedling,  still  older,  bearing  a  well-developed 
leaf,  had  a  sharp  rigid  filament  affixed  to  one  of  its  cotyledons 
(85  mm.  in  length),  which  recorded  its  own  movements  on 
a  revolving  drum  with  smoked  paper.  The  observations  were 
made  in  the  hot-house,  where  the  plant  had  lived,  so  that  there 
was  no  change  in  temperature  or  light.  The  record  commenced 
at  11  A.M.  on  February  18th ;  and  from  this  hour  till  3  P.M.  the 


42  CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 

cotyledon  fell;  it  then  rose  rapidly  till  9  P.M.,  then  very 
gradually  till  3  A.M.  February  19th,  after  which  hour  it  sank 
gradually  till  4.30  P.M.  ;  but  the  downward  movement  was  inter- 
rupted by  one  slight  rise  or  oscillation  about  1.30  P.M.  After 
4.30  P.M.  (19th)  the  cotyledon  rose  till  1  A.M.  (in  the  night  of 
February  20th)  and  then  sank  very  gradually  till  9.30  A.M., 
when  our  observations  ceased.  The  amount  of  movement  was 
greater  on  the  18th  than  on  the  19th  or  on  the  morning  of 
the  20th. 

Cucurbita  aurantia. — An  arched  hypocotyl  was  found  buried  a 
little  beneath  the  surface  of  the  soil ;  and  in  order  to  prevent  it 
straightening  itself  quickly,  when  relieved  from  the  surrounding 
pressure  of  the  soil,  the  two  legs  of  the  arch  were  tied  together. 
The  seed  was  then  lightly  covered  with  loose  damp  earth.  A 
filament  with  a  bead  at  the  end  was  affixed  to  the  basal  leg,  the 
movements  of  which  were  observed  during  two  days  in  the 
usual  manner.  On  the  first  day  the  arch  moved  in  a  zigzag  line 
towards  the  side  of  the  basal  leg.  On  the  next  day,  by  which 
time  the  dependent  cotyledons  had  been  dragged  above  the  sur- 
face of  the  soil,  the  tied  arch  changed  its  course  greatly  nine 
times  in  the  course  of  14£  h.  It  swept  a  large,  extremely  irre- 
gular, circular  figure,  returning  at  night  to  nearly  the  same 
spot  whence  it  had  started  early  in  the  morning.  The  line  was 
so  strongly  zigzag  that  it  apparently  represented  five  ellipses,  with 
their  longer  axes  pointing  in  various  directions.  With  respect 
to  the  periodical  movements  of  the  cotyledons,  those  of  several 
young  seedlings  formed  together  at  4  P.M.  an  angle  of  about  60°, 
and  at  10  P.M.  their  lower  parts  stood  vertically  and  were  in 
contact ;  their  tips,  however,  as  is  usual  in  the  genus,  were  per- 
manently reflexed.  These  cotyledons,  at  7  A.M.  on  the  following 
morning,  were  again  well  expanded. 

Layenaria  vulgaris  (var.  miniature  Bottle-gourd)  (Cucurbi- 
taceae).— A  seedling  opened  its  cotyledons,  the  movements  of 
which  were  alone  observed,  slightly  on  June  27th,  and  closed 
them  at  night:  next  day,  at  noon  (28th),  they  included  an 
angle  of  53°,  and  at  10  P.M.  they  were  in  close  contact,. so  that 
each  had  risen  261°.  At  noon,  on  the  29th,  they  included  an 
angle  of  118°,  and  at  10  P.M.  an  angle  of  54°,  so  each  had 
risen  32°.  On  the  following  day  they  were  still  more  open,  and 
the  nocturnal  rise  was  greater,  but  the  angles  were  not  measured. 
Two  other  seedlings  were  observed,  and  behaved  during  three 
days  in  a  closely  similar  manner.  The  cotyledons,  therefore, 


CHAP.  I. 


CUCUKBITA. 


10°35f.m 


open  more  and  more  on  each  succeeding  day,  and  rise  each 
night  about  30° ;  consequently  during  the  first  two  nights  of 
their  life  they  stand  vertically  and 
come  into  contact. 

In  order  to  ascertain  more  ac- 
curately the  nature  of  these  move- 
ments, the  hypocotyl  of  a  seedling, 
with  its  cotyledons  well  expanded, 
was  secured  to  a  little  stick,  and  a 
filament  with  triangles  of  paper 
was  affixed  to  one  of  the  cotyledons. 
The  observations  were  made  under 
a  rather  dim  skylight,  and  the 
temperature  during  the  whole  time 
was  between  17£°  to  18°  C.  (63°  to 
65°  F.).  Had  the  temperature  been 
higher  and  the  light  brighter,  the 
movements  would  probably  have 
been  greater.  On  July  llth  (see 
Fig.  30),  the  cotyledon  fell  from 
7.35  A.M.  till  10  A.M.  ;  it  then  rose 
(rapidly  after  4  P.M.)  till  it  stood 
quite  vertically  at  8.40  P.M.  During 
the  early  morning  of  the  next  day 
(12th)  it  fell,  and  continued  to  fall 
till  8  A.M.,  after  which  hour  it  rose, 
then  fell,  and  again  rose,  so  that  by 
10.35  P.M.  it  stood  much  higher  than 
it  did  in  the  morning,  but  was  not 
vertical  as  on  the  preceding  night. 
During  the  following  early  morn- 
ing and  whole  day  (13th)  it  fell  and 
circumnutated,  but  had  not  risen 
when  observed  late  in  the  evenirfg ; 
and  this  wa&  probably  due  to  the 
deficiency  of  heat  or  light,  or  of 
both.  "We  thus  see  that  the  coty- 
ledons became  more  widely  open  at 
noon  on  each  succeeding  day ;  and 
that  they  rose  considerably  each  night,  though  not  acquiring 
a  vertical  position,  except  during  the  first  two  nights. 

Cucumis  dudaim  (Cucurbitaceae).— Two  seedlings  had  opened 


Lagcnaria  vulgaris :  circumnu- 
tation  of  a  cotyledon,  l£ 
inch  in  length,  apex  only  4f 
inches  from  the  vertical  glass, 
on  which  its  movements  were 
traced  from  7.35  A.M.  July 
llth  to  9.5  A.M.  on  the  14th. 
Figure  here  given  reduced 
to  one-third  of  original  scale. 


M  CIRCUMNUTATION  OF  SEEDLINGS.         CHAP.  I. 

their  cotyledons  for  the  first  time  during  the  day, — one  to  the 
extent  of  90°  and  the  other  rather  more;  they  remained  in 
nearly  the  same  position  until  10.40  P.M.  ;  but  by  7  A.M.  on  the 
following  morning  the  one  which  had  been  previously  open  to 
the  extent  of  90°  had  its  cotyledons  vertical  and  completely 
shut ;  the  other  seedling  had  them  nearly  shut.  Later  in  the 
morning  they  opened  in  the  ordinary  manner.  It  appears 
therefore  that  the  cotyledons  of  this  plant  close  and  open  at 
somewhat  different  periods  from  those  of  the  foregoing  species 
of  the  allied  genera  of  Cucurbita  and  Lagenaria. 

Opuntia,  basilaris  (Cacteae). — A  seedling  was  carefully  ob- 
served,   because     considering     its 
Fig.  31.  appearance  and  the  nature  of  the 

mature  plant,  it  seemed  very  un- 
likely that  either  the  hypocotyl  or 
cotyledons  would  circumnutate  to 
an  appreciable  extent.  The  coty- 
ledons were  well  developed,  being 
•9  of  an  inch  in  length,  "22  in 
breadth,  and  *15  in  thickness. 
The  almost  cylindrical  hypocotyl, 
now  bearing  a  minute  spinous  bud 

/on  its  summit,  was  only  '45  of  an 
inch  in  height,  and  '19  in  dia- 

Opuntia  basilaris:  conjoint  cir-  meter'  The  tracing  (Fig.  31)  shows 
cumnutation  of  hypocotyl  the  combined  movement  of  the 
and  cotyledon ;  filament  hypocotyl  and  of  one  of  the  coty- 
fixed  longitudinally  to  coty-  led  frQm  4  45  p  M>  Qn  M  2gth 
ledon,  and  movement  traced  .  ,,  .,  01  , 

during  66  h.  on  horizontal  to  11  A.M.  onthe  31st.  On  the  29th 
glass.  Movement  of  the'ter-  a  nearly  perfect  ellipse  was  com- 
minal  bead  magnified  about  pleted  On  the  30th  the  hypocotyl 
30  times,  here  reduced  to  one-  -,  - 

third  scale.    Seedling  kept  in    moved>  from  SOm°  ™known  Cause, 

hot-house,  feebly  illuminated   in  the  same  general  direction  in  a 

from  above.  zigzag  line ;  but  between  4.30  and 

10  P.M.  almost  completed  a  second 

small  ellipse.  The  cotyledons  move  only  a  little  up  and  down  : 
thus  at  10.15  P.M.  they  stood  only  10°  higher  than  at  noon.  The 
chief  seat  of  movement  therefore,  at  least  when  the  cotyledons 
are  rather  old  as  in  the  present  case,  lies  in  the  hypocotyl.  The 
ellipse  described  on  the  29th  had  its  longer  axis  directed  at 
nearly  right  angles  to  a  line  joining  the  two  cotyledons.  The 
actual  amount  of  movement  of  the  bead  at  the  end  of  the 


CHAP.  I. 


PEIMULA. 


45 


filament  was,  as  far  as  could  be  ascertained,  about  "14  of  an 
inch. 

Hclianthus  annuus  (Composite). — The  upper  part  of  the 
hypocotyl  moved  during  the 
day-time  in  the  course 
shown  in  the  annexed  figure 
(Fig.  32).  As  the  line  runs 
in  various  directions,  cross- 
ing itself  several  times, 
the  movement  may  be  con- 
sidered as  one  of  circumnu- 
tation. The  extreme  actual 
distance  travelled  was  at 
least  *1  of  an  inch.  The 
movements  of  the  cotyle- 
dons of  two  seedlings  were  Helianthus  annuus :  circumnutation  of 
-,  ,,  .  hypocotyl,  with  filament  fixed  across 

observed;  onefacmg  a  north-  its  summit,  traced  on  a  horizontal 
east  window,  and  the  other  glass  in  darkness,  from  8.45  A.M.  to 
so  feebly  illuminated  from  10-4'5  P-M-' and  for  an  hour  on  f°llow- 

ibnvP    a<?    tn    bp     ilmnsf    in        in§    morning-      Movement    of   bead 
above    as    to    be    almost   in        magnified  21  times,  here  reduced  to 
darkness.     They  continued      one-half  of  original  scale. 
to    sink    till   about    noon, 

when  they  began  to  rise;  but  between  5  and  7  or  8  P.M. 
they  either  sank  a  little,  or  moved  laterally,  and  then  again 
began  to  rise.  At  7  A.M.  on  the  following  morning  those  on 
the  plant  before  the  north-east  window  had  opened  so  little 
that  they  stood  at  an  angle  of  73°  above  the  horizon,  and  were 
not  observed  any  longer.  Those  on  the  seedling  which  had 
been  kept  in  almost  complete  darkness,  sank  during  the  whole 
day,  without  rising  about  mid-day,  but  rose  during  the  night. 
On  the  third  and  fourth  days  they  continued  sinking  without 
any  alternate  ascending  movement;  and  this,  no  doubt,  was 
due  to  the  absence  of  light. 

Primula  Sinensis  (Primulacese). — A  seedling  was  placed  with 
the  two  cotyledons  parallel  to  a  north-east  window  on  a  day 
when  the  light  was  nearly  uniform,  and  a  filament  was  affixed 
to  one  of  them.  From  observations  subsequently  made  on 
another  seedling  with  the  stem  secured  to  a  stick,  the  greater 
part  of  the  movement  shown  in  the  annexed  figure  (Fig.  33), 
must  have  been  that  of  the  hypocotyl,  though  the  cotyledons 
certainly  move  up  and  down  to  a  certain  extent  both  during  the 
day  and  night.  The  movements  of  the  same  seedling  were  traced 


CIRCUMNUTATION   OF   SEEDLINGS. 


CHAP.  1 


on  the  following  day  with  nearly  the  same  result;  and  there 
can  be  no  doubt  about  the  circumnutation  of  the  hypocotyl. 


Primula  Sinensis :  conjoint  circumnntation  of  hypocotyl  and  cotyledon, 
traced  on  vertical  glass,  from  8.40  A.M.  to  10.45  P.M.  Movements  of 
bead  magnified  about  26  times. 

Cyclamen  Persicum  (Primulacese). — This  plant  is  generally  sup- 
posed to  produce  only  a  single  cotyledon,  but  Dr.  H.  Gressner  * 
has  shown  that  a  second  one  is  developed  after  a  long  interval 
of  time.  The  hypocotyl  is  converted  into  a  globular  conn,  even 
before  the  first  cotyledon  has  broken  through  the  ground  with  its 
blade  closely  enfolded  and  with  its  petiole  in  the  form  of  an  arch, 
like  the  arched  hypocotyl  or  epicotyl  of  any  ordinary  dicotyle- 
donous plant.  A  glass  filament  was  affixed  to  a  cotyledon,  '55 
of  an  inch  in  height,  the  petiole  of  which  had  straightened  itself 
and  stood  nearly  vertical,  but  with  the  blade  not  as  yet  fully 
expanded.  Its  movements  were  traced  during  24£  h.  on  a 

horizontal  glass^  magnified  50 
times ;  and  in  this  interval  it 
described  two  irregular  small 
circles;  it  therefore  circumnu- 
tates,  though  on  an  extremely 
small  scale. 

Stapelia  sarpedon  (Ascle- 
piadeao).  —  This  plant,  when 
mature,  resembles  a  cactus. 
The  flattened  hypocotyl  is 


Stapelia  sarpedon :  circumnutation 
of  hypocotyl,  illuminated  from 
above,  traced  on  horizontal  glass, 
from  6.45  A.M.  June  26th  to  8.45 
A.M.  28th.  Temp.  23°-24°  C. 
Movement  of  bead  magnified  21 
times 


fleshy,  enlarged  in  the  upper 
part,  and  bears  two  rudimen- 
tary cotyledons.  It  breaks 

through  the  ground  in  an  arched  form,  with  the  rudimentary 
cotyledons  closed  or  in  contact.    A  filament  was  affixed  almost 


*  4Bot.  Zeitung,'  3874,  p.  837. 


CHAP.  I.  IPOMCEA.  47 

vertically  to  the  hypocotyl  of  a  seedling  half  an  inch  high ;  and 
its  movements  were  traced  during  50  h.  on  a  horizontal  glass 
(Fig.  34).  From  some  unknown  cause  it  bowed  itself  to  one 
side,  and  as  this  was  effected  by  a  zigzag  course,  it  probably 
circumnutated ;  but  with  hardly  any  other  seedling  observed 
by  us  was  this  movement  so  obscurely  shown. 

Ipomcea  ccerulea  vel  Pharbitis  nil  (Convolvulaceae). — Seedlings 
of  this  plant  were  observed  because  it  is  a  twiner,  the  upper 
interned es  of  which  circumnutate  conspicuously;  but,  like 
other  twining  plants,  the  first  few  internodes  which  rise  above 
the  ground  are  stiff  enough  to  support  themselves,  and  therefore 
do  not  circumnutate  in  any  plainly  recognisable  manner.*  In 
this  particular  instance  the  fifth  internode  (including  the  hypo- 
cotyl) was  the  first  which  plainly  circumnutated  and  twined 
round  a  stick.  We  therefore  wished  to  learn  whether  circuni- 
nutation  could  be  observed  in  the  hypocotyl  if  carefully  observed 
in  our  usual  manner.  Two  seedlings  were  kept  in  the  dark 
with  filaments  fixed  to  the  upper  part  of  their  hypocotyls ;  but 
from  circumstances  not  worth  explaining  their  movements  were 
traced  for  only  a  short  time.  One  moved  thrice  forwards  and 
twice  backwards  in  nearly  opposite  directions,  in  the  course  of 
3  h.  15  m. ;  and  the  other  twice  forwards  and  twice  backwards 
in  2  h.  22  m.  The  hypocotyl  therefore  circumnutated  at  a  re- 
markably rapid  rate.  It  may  here  be  added  that  a  filament  was 
affixed  transversely  to  the  summit  of  the  second  internode  above 
the  cotyledons  of  a  little  plant  3£  inches  in  height;  and  its 
movements  were  traced  on  a  horizontal  glass.  It  circumnutated, 
and  the  actual  distance  travelled  from  side  to  side  was  a  quarter 
of  an  inch,  which  was  too  small  an  amount  to  be  perceived  with- 
out the  aid  of  marks. 

The  movements  of  the  cotyledons  are  interesting  from  their 
complexity  and  rapidity,  and  in  some  other  respects.  The 
hypocotyl  (2  inches  high)  of  a  vigorous  seedling  was  secured  to  a 
stick,  and  a  filament  with  triangles  of  paper  was  affixed  to  one 
of  the  cotyledons.  The  plant  was  kept  all  day  in  the  hot-house, 
and  at  4.20  P.M.  (June  20th)  was  placed  under  a  skylight  in 
the  house,  and  observed  occasionally  during  the  evening  and 
night.  It  fell  in  a  slightly  zigzag  line  to  a  moderate  extent 
from  4.20  P.M.  till  10.15  P.M.  When  looked  at  shortly  after  mid- 
night (12.30  P.M.)  it  had  risen  a  very  little,  and  considerably  by 

*  « Movements  and  Habits  of  Climbing  Plants,*  p.  33,  1875. 


48 


CIRCUMNUTATION  OF  SEEDLINGS.        CHAP.  I 


3.45  A.M.    When  again  looked 
Fig.  35. 


Ipomcea  ccerulea  :  circumnutation  of 
cotyledon,  traced  on  vertical  glass, 
from  6.10  A.M.  June  21st  to  6.45 
A.M.  22nd.  Cotyledon  with  petiole 
1'6  inch  in  length,  apex  of  blade 
4'1  inch  from  the  vertical  glass; 
so  movement  not  greatly  mag- 
nified;  temp.  20°  C. 


at,  at  6.10  A.M.  (21st),  it  had 
fallen  largely.  A  new  tracing 
was  now  begun  (see  Fig.  35), 
and  soon  afterwards,  at  6.42 
A.M.,  the  cotyledon  had  risen  a 
little.  During  the  forenoon  it 
was  observed  about  every 
hour;  but  between  12.30  and 
6  P.M.  every  half-hour.  If  the 
observations  had  been  made  at 
these  short  intervals  during  the 
whole  day,  the  figure  would 
have  been  too  intricate  to  have 
been  copied.  As  it  was,  the 
cotyledon  moved  up  and  down 
in  the  course  of  16  h.  20  m.  (i.e. 
between  6.10  A.M.  and  10.30 
P.M.)  thirteen  times. 

The  cotyledons  of  this  seed- 
ling sank  downwards  during 
both  evenings  and  the  early 
part  of  the  night,  but  rose 
during  the  latter  part.  As  this 
is  an  unusual  movement,  the 
cotyledons  of  twelve  other  seed- 
lings were  observed ;  they  stood 
almost  or  quite  horizontally  at 
mid-day,  and  at  10  P.M.  were 
all  declined  at  various  angles. 
The  most  usual  angle  was  be- 
tween 30°  and  35°;  but  three 
stood  at  about  50°  and  one  at 
even  70°  beneath  the  horizon. 
The  blades  of  all  these  cotyle- 
dons had  attained  almost  their 
full  size,  viz.  from  1  to  1£  inches 
in  length,  measured  along  their 
midribs.  It  is  a  remarkable 
fact  that  whilst  young— that 
is,  when  less  than  half  an  inch 
in  length,  measured  in  the 
same  manner— they  do  not  sink 


CHAP.  T.  CERINTHE.  49 

downwards  in  the  evening.  Therefore  their  weight,  which  is 
considerable  when  almost  fully  developed,  probably  came  into 
play  in  originally  determining  the  downward  movement.  The 
periodicity  of  this  movement  is  much  influenced  by  the  degree 
of  light  to  which  the  seedlings  have  been  exposed  during  the 
day;  for  three  kept  in  an  obscure  place  began  to  sink  about 
noon,  instead  of  late  in  the  evening ;  and  those  of  another  seed- 
ling were  almost  paralysed  by  having  been  similarly  kept  during 
two  whole  days.  The  cotyledons  of  several  other  species  of 
Ipomoea  likewise  sink  downwards  late  in  the  evening. 

Cerinthe    major    (Boraginese). — The   circumnutation    of  the 
hypocotyl  of   a   young  seedling  with  the  cotyledons  hardly 

Fig.  36. 


Cerinthe  major:  circumnutation  of  hypocotyl,  with  filament  fixed  across  its 
summit,  illuminated  from  above,  traced  on  horizontal  glass,  from 
9.26  A.M.  to  9.53  P.M.  on  Oct.  25th.  Movement  of  the  bead  magnified 
30  times,  here  reduced  to  one-third  of  original  scale. 

expanded,  is  shown  in  the  annexed  figure  (Fig.  36),  which 
apparently  represents  four  or  five  irregular  ellipses,  described 
in  the  course  of  a  little  over  12  hours.  Two  older  seedlings 
were  similarly  observed,  excepting  that  one  of  then!  was  kept 
in  the  dark ;  their  hypocotyls  also  circumnutated,  but  in  a  more 
simple  manner.  The  cotyledons  on  a  seedling  exposed  to  the 
light  fell  from  the  early  morning  until  a  little  after  noon,  and 
then  continued  to  rise  until  10.30  P.M.  or  later.  The  cotyledons 
of  this  same  seedling  acted  in  the  same  general  manner  during 
the  two  following  days.  It  had  previously  been  tried  in  the 
dark,  and  after  being  thus  kept  for  only  1  h.  40  m.  the  cotyledons 
began  at  4.30  P.M..  to  sink,  instead  of  continuing  to  rise  till  late 
at  night. 


50  CIECUMNUTATION  OF  SEEDLINGS.         CHAP.  L 

Nolana  prostrata  (Nolaneae).  —  The  movements  were  not 
traced,  but  a  pot  with  seedlings,  which  had  been  kept  in  the 
dark  for  an  hour,  was  placed  tinder  the  microscope,  with  the 
micrometer  eye-piece  so  adjusted  that  each  division  equalled 
•g^o-th  of  an  inch.  The  apex  of  one  of  the  cotyledons  crossed 
rather  obliquely  four  divisions  in  13  minutes  ;  it  was  also  sink- 
ing, as  shown  by  getting  out  of  focus.  The  seedlings  were 
again  placed  in  darkness  for  another  hour,  and  the  apex  now 
crossed  two  divisions  in  6  m.  18  s.  ;  that  is,  at  very  nearly  the 
same  rate  as  before.  After  another  interval  of  an  hour  in  dark- 
ness, it  crossed  two  divisions  in  4  m.  15  s.,  there- 
Fig.  37.  fore  at  a  quicker  rate.  In  the  afternoon,  after  a 
longer  interval  in  the  dark,  the  apex  was  motion- 
less, but  after  a  time  it  recommenced  moving, 
though  slowly  ;  perhaps  the  room  was  too  cold. 
Judging  from  previous  cases,  there  can  hardly 
be  a  doubt  that  this  seedling  was  circumnuta- 
ting. 

Solatium  ly  coper  sicum  (Solanese).  —  The  move- 
ments of  the  hypocotyls  of  two  seedling  to- 
Solanum  ly  coper-  matoes  were  observed  during  seven  hours,  and 
sicum:  circum-  there  could  be  no  doubt  that  both  circumnu- 
n«*»tion  ofjiy-  tated<  They  were  illuminated  from  above,  but 
filament  fixed  ^v  an  accident  a  little  light  entered  on  one  side, 
across  its  sum-  and  in  the  accompanying  figure  (Fig.  37)  it 
mit,  traced  on  may  be  seen  that  the  hypocotyl  moved  to  this 

fromTo  A.M*  to  side  (the  ^PP6*  one  in  the  fiSure)>  making  small 
5  P.M.  Oct.  24th.  loops  and  zigzagging  in  its  course.  The  move- 
Illuminated  ob-  ments  of  the  cotyledons  were  also  traced  both 
liquely  from  on  verticai  and  horizontal  glasses  ;  their  angles 
me°nte'  Of  ^ead  wittl  the  h°rizon  were  likewise  measured  at 


magnified  about  various  hours.    They  fell  from  8.30  A.M.  (October 
35  times,  here   17th)  to  about  noon  ;  then  moved  laterally  in  a 

So?  original    ziSzag  line>and  at  ab°ut  4P'M-  beSant°  ris6' 
scale<  they  continued  to  do  so  until  10.30  P.M.,   by 

which  hour  they  stood  vertically  and  were  asleep. 
At  what  hour  of  the  night  or  early  morning  they  began  to  fall 
was  not  ascertained.  Owing  to  the  lateral  movement  shortly 
after  mid-day,  the  descending  and  ascending  lines  did  not 
coincide,  and  irregular  ellipses  were  described  during  each  24  h. 
The  regular  periodicity  of  these  movements  is  destroyed,  as  wo 
shall  hereafter  see,  if  the  seedlings  are  kept  in  the  dark. 


CHAP.  I.  SOLANUM.  51 

Solanum  palinacanthum. — Several  arched  hypocotyls  rising 
nearly  '2  of  an  inch  above  the  ground,  but  with  the  cotyledons 
still  buried  beneath  the  surface,  were  observed,  and  the  tracings 
showed  that  they  circumnutated. .  Moreover,  in  several  cases 
little  open  circular  spaces  or  cracks  in  the  argillaceous  sand 
which  surrounded  the  arched  hypocotyls  were  visible,  and 
these  appeared  to  have  been  made  by  the  hypocotyls  having 
bent  first  to  one  and  then  to  another  side  whilst  growing  up- 
wards. In  two  instances  the  vertical  arches  were  observed  to 
move  to  a  considerable  distance  backwards  from  the  point  where 
the  cotyledons  lay  buried;  this  movement,  which  has  been 
noticed  in  some  other  cases,  and  which  seems  to  aid  in  extracting 
the  cotyledons  from  the  buried  seed-coats,  is  due  to  the  com- 
mencement of  the  straightening  of  the  hypocotyl.  In  order  to 
prevent  this  latter  movement,  the  two  legs  of  an  arch,  the 

V  Fig.  38. 


Solanum  palinacantltum :  circumnutation  of  an  arched  hypocotyl,  just 
emerging  from  the  ground,  with  the  two  legs  tied  together,  traced  in 
darkness  on  a  horizontal  glass,  from  9.20  A.M.  Dec.  17th  to  8.30  A.M. 
19th.  Movement  of  bead  magnified  13  times;  but  the  filament,  which 
was  affixed  obliquely  to  the  crown  of  the  arch,  was  of  unusual  length. 

summit  of  which  was  on  a  level  with  the  surface  of  the  soil, 
were  tied  together ;  the  earth  having  been  previously  removed 
to  a  little  depth  all  round.  The  movement  of  the  arch  during 
47  hours  under  these  unnatural  circumstances  is  exhibited 
in  the  annexed  figure. 

The  cotyledons  of  some  seedlings  in  the  hot-house  were  hori- 
zontal about  noon  on  December  13th ;  and  at  10  P.M.  had  risen 
to  an  angle  of  27°  above  the  horizon ;  at  7  A.M.  on  the  following 


52 


CIKCUMNUTATION  OF  SEEDLINGS.        CHAP.  I. 


Fig.  39. 


morning,  before  it  was  light,  they  had  risen  to  59°  above  tho 
horizon;  in  the  afternoon  of  the  same  day  they  were  found 
again  horizontal. 

Beta  vulgaris  (Chenopodese). — The  seedlings  are  excessively 
sensitive  to  light,  so  that  although  on  the  first  day  they 
were  uncovered  only  during  two  or  three 
minutes  at  each  observation,  they  all  moved 
steadily  towards  the  side  of  the  room 
whence  the  light  proceeded,  and  the  trac- 
ings consisted  only  of  slightly  zigzag  lines 
directed  towards  the  light.  On  the  next 
day  the  plants  were  placed  in  a  completely 
darkened  room,  and  at  each  observation 
were  illuminated  as  much  as  possible  from 
vertically  above  by  a  small  wax  taper.  The 
annexed  figure  (Fig.  39)  shows  the  move- 
ment of  the  hypocotyl  during  9  h.  under 
these  circumstances.  A  second  seedling 
was  similarly  observed  at  the  same  time, 
and  the  tracing  had  the  same  peculiar 
character,  due  to  the  hypocotyl  often  mov- 
ing and  returning  in  nearly  parallel  lines. 
The  movement  of  a  third  hypocotyl  differed 
greatly. 

We  endeavoured  to  trace  the  movements 
of  the  cotyledons,  and  for  this  purpose 
some  seedlings  were  kept  in  the  dark,  but 
they  moved  in  an  abnormal  manner ;  they 
continued  rising  from  8.45  A.M.  to  2  P.M., 
then  moved  laterally,  and  from  3  to  6  P.M. 
descended ;  whereas  cotyledons  which  have  been  exposed  all 
the  day  to  the  light  rise  in  the  evening  so  as  to  stand  verti- 
cally at  night;  but  this  statement  applies  only  to  young 
seedlings.  For  instance,  six  seedlings  in  the  greenhouse  had 
their  cotyledons  partially  open  for  the  first  time  on  the  morning 
of  November  15th,  and  at  8.45  P.M.  all  were  completely  closed, 
so  that  they  might  properly  be  said  to  be  asleep.  Again,  on  the 
morning  of  November  27th,  the  cotyledons  of  four  other  seedlings, 
which  were  surrounded  by  a  collar  of  brown  paper  so  that  they 
received  light  only  from  above,  were  open  to  the  extent  of 
39°;  at  10  P.M.  they  were  completely  closed;  next  morning 
(November  28th)  at  6.45  A.M.,  whilst  it  was  still  dark,  two  of  them 


Beta  vu'garis:  circum- 
nutation  of  hypo- 
cotyl, with  filament 
fixed  obliquely  a- 
cross  its  summit, 
traced  in  darkness 
on  horizontal  glass, 
from  8.25  A.M.  to 
5.30  P.M.  Nov.  4th. 
Movement  of  bead 
magnified  23  times, 
here  reduced  to  one- 
third  of  original 
scale. 


CHAP.  I.  RICINUS    AND    QUERCUS.  53 

were  partially  open  and  all  opened  in  the  course  of  the  morning ; 
but  at  10.20  P.M.  all  four  (not  to  mention  nine  others  which 
had  been  open  in  the  morning  and  six  others  on  another  occa- 
sion) were  again  completely  closed.  On  the  morning  of  the 
29th  they  were  open,  but  at  night  only  one  of  the  four  was 
closed,  and  this  only  partially;  the  three  others  had  their 
cotyledons  much  more  raised  than  during  the  day.  On  the 
night  of  the  30th  the  cotyledons  of  the  four  were  only  slightly 
raised. 

Ricinus  Borboniensis  (Euphorbiacese).— Seeds  were  purchased 
under  the  above  name — probably  a  variety  of  the  common  castor- 
oil  plant.  As  soon  as  an  arched  hypocotyl  had  risen  clear  above 
the  ground,  a  filament  was  attached  to  the  upper  leg  bearing  the 
cotyledons  which  were  still  buried  beneath  the  surface,  and  the 
movement  of  the  bead  was  traced  on  a  horizontal  glass  during 
a  period  of  34  h.  The  lines  traced  were  strongly  zigzag,  and 
as  the  bead  twice  returned  nearly  parallel  to  its  former  course 
in  two  different  directions,  there  could  be  no  doubt  that  the 
arched  hypocotyl  circumnutated.  At  the  close  of  the  34  h. 
the  upper  part  began  to  rise  and  straighten  itself,  dragging  the 
cotyledons  out  of  the  ground,  so  that  the  movements  of  the 
bead  could  no  longer  be  traced  on  the  glass. 

Quercus  (American  sp.)  (Cupuliferse). — Acorns  of  an  American 
oak  which  had  germinated  at  Kew  were  planted  in  a  pot  in 
the  greenhouse.  This  transplantation  checked  their  growth; 
but  after  a  time  one  grew  to  a  height  of  five  inches, 
measured  to  the  tips  of  the  small  partially  unfolded  leaves  on 
the  summit,  and  now  looked  vigorous.  It  consisted  of  six 
very  thin  internodes  of  unequal  lengths.  Considering  these 
circumstances  and  the  nature  of  the  plant,  we  hardly  expected 
that  it  would  circumnutate ;  but  the  annexed  figure  (Fig.  40) 
shows  that  it  did  so  in  a  conspicuous  manner,  changing  its 
course  many  times  and  travelling  in  all  directions  during  the 
48  h.  of  observation.  The  figure  seems  to  represent  5  or  6 
irregular  ovals  or  ellipses.  The  actual  amount  of  movement 
from  side  to  side  (excluding  one  great  bend  to  the  left)  was 
about  *2  of  an  inch ;  but  this  was  difficult  to  estimate,  as  owing 
to  the  rapid  growth  of  the  stem,  the  attached  filament  was 
much  further  from  the  mark  beneath  at  the  close  than  at  the 
commencement  of  the  observations.  It  deserves  notice  that  the 
pot  was  placed  in  a  north-east  room  within  a  deep  box,  the  top 
of  which  was  not  at  first  covered  up,  so  that  the  inside  facing 


54  CIRCUMNUTATION   OF   SEEDLINGS.         CHAP.  I. 

the  windows  was  a  little  more  illuminated  than  the  opposite 
side;  and  during  the  first  morning  the  stem  travelled  to  a 
greater  distance  in  this  direction  (to  the  left  in  the  figure)  than 
it  did  afterwards  when  the  box  was  completely  protected  from 
light 

Fig.  40. 


Quercus  (American  sp.)  :  circumnutation  of  young  stem,  traced  on  hori- 
zontal glass,  from  12.50  P.M.  Feb.  22nd  to  12.50P.M.  24th.  Movement 
of  bead  greatly  magnified  at  first,  but  slightly  towards  the  close  of  the 
observations — about  10  times  on  an  average. 

Quercus  rdbur. — Observations  were  made  only  on  the  move- 
ments of  the  radicles  from  germinating  acorns,  which  were  allowed 
to  grow  downwards  in  the  manner  previously  described,  over 
plates  of  smoked  glass,  inclined  at  angles  between  65°  and  69° 
to  the  horizon.  In  four  cases  the  tracks  left  were  almost  straight, 
but  the  tips  had  pressed  sometimes  with  more  and  sometimes 
with  less  force  on  the  glass,  as  shown  by  the  varying  thickness 
of  the  tracks  and  by  little  bridges  of  soot  left  across  them. 
In  the  fifth  case  the  track  was  slightly  serpentine,  that  is,  the 
tip  had  moved  a  little  from  side  to  side.  In  the  sixth  case 
(Fig.  41,  A)  it  was  plainly  serpentine,  and  the  tip  had  pressed 
almost  equably  on  the  glass  in  its  whole  course.  In  the  seventh 
case  (B)  the  tip  had  moved  both  laterally  and  had  pressed 


CHAP.  I. 


QUEECUS  AND  COKYLUS. 


55 


alternately  with  unequal  force  on  the  glass;  so  that  it  had 
moved  a  little  in  two  planes  at  right  angles  to  one  another.  In 
the  eighth  and  last  case  (C)  it  had  moved  very  little  laterally, 
but  had  alternately  left  the  glass  and  come  into  contact  with  it 
again.  There  can  be  no  doubt  that  in  the  last  four  cases  the 
ladicle  of  the  oak  circuinnutatcd  whilst  growing  downwards. 

Fig.  41. 


B 


Quercus  robur :  tracks  left  on  inclined  smoked  glass-plates  by  tips  of 
radicles  in  growing  downwards.  Plates  A  and  C  inclined  at  65°  and 
plate  B  at  68°  to  the  horizon. 

Corylus  avellana  (Corylacese).— The  epicotyl  breaks  through 
the  ground  in  an  arched  form ;  but  in  the  specimen  which  was 
first  examined,  the  apex  had  become  decayed,  and  the  epicotyl 
grew  to  some  distance  through  the  soil,  in  a  tortuous,  almost 
horizontal  direction,  like  a  root.  In  consequence  of  this  injury 
it  had  emitted  near  the  hypogean  cotyledons  two  secondary 
shoots,  and  it  was  remarkable  that  both  of  these  were  arched, 
like  the  normal  epicotyl  in  ordinary  cases.  The  soil  was  removed 
from  around  one  of  these  arched  secondary  shoots,  and  a  glass 
filament  was  affixed  to  the  basal  leg.  The  whole  was  kept 
damp  beneath  a  metal-box  with  a  glass  lid,  and  was  thus  illumi- 
nated only  from  above.  Owing  apparently  to  the  lateral  pressure 
of  the  earth  being  removed,  the  terminal  and  bowed-down  part 
of  the  shoot  began  at  once  to  move  upwards,  so  that  after 
'24  h.  it  formed  a  right  angle  with  the  lower  part.  This  lower 
part,  to  which  the  filament  was  attached,  also  straightened 
itself,  and  moved  a  little  backwards  from  the  upper  part.  Con- 
sequently a  long  line  was  traced  on  the  horizontal  glass ;  and 


56 


CIKCUMNUTATION  OF  SEEDLINGS.        CHAP.  L 


this  was   in  parts   straight    and   in  parts  decidedly  zigzag, 

indicating  circumnutation. 

On  the  following  day  the  other  secondary  shoot  was  observed ; 

it  was  a  little  more  advanced  in  age,  for  the  upper  part,  instead 
of  depending  vertically  downwards, 
stood  at  an  angle  of  45°  above  the 
horizon.  The  tip  of  the  shoot  pro- 
jected obliquely  '4  of  an  inch  above 
the  ground,  but  by  the  close  of  our 
observations,  which  lasted  47  h.,  it 
had  grown,  chiefly  towards  its  base, 
to  a  height  of  •  85  of  an  inch.  The 
filament  was  fixed  transversely  to 
the  basal  and  almost  upright  half 
of  the  shoot,  close  beneath  the  lowest 
scale-like  appendage.  The  circum- 
nutating  course  pursued  is  shown 
in  the  accompanying  figure  (Fig. 
42).  The  actual  distance  traversed 
from  side  to  side  was  about  '04  of 
an  inch. 

Pinus  pinaster  (Coniferae).  —  A 
young  hypocotyl,  with  the  tips 
of  the  cotyledons  still  enclosed 
within  the  seed-coats,  was  at  first 

only  *35  of  an  inch  in  height;    but  the  upper  part  grew  so 

rapidly  that  at  the  end  of  our  observations  it  was  •  6  in  height, 


\ 

Corylus  avellana:  circumnuta- 
tion of  a  young  shoot  emitted 
from  the  epicotyl,  the  apex 
of  which  had  been  injured, 
traced  on  a  horizontal  glass, 
from  9  A.M.  Feb.  2ud  to  8 
A.M.  4th.  Movement  ol 
bead  magnified  about  27 
times. 


Fig.  43. 


Pinm  pinaster :  circumnutation  of  hypocotyl,  with  filament  fixed  across  its 
summit,  traced  on  horizontal  glass,  from  10  A.M.  March  21st  to  9  A.M. 
23rd.  Seedling  kept  in  darkness.  Movement  of  bead  magnified  about 
35  times. 


CHAF.  I.  PINUS  AND  CYCAS.  57 

and  by  this  time  the  filament  was  attached  some  way  down  the 
little  stem.  From  some  unknown  cause,  the  hypocotyl  moved 
far  towards  the  left,  but  there  could  be  no  doubt  (Fig.  43)  that 
it  circumnutated.  Another  hypocotyl  was  similarly  observed, 
and  it  likewise  moved  in  a  strongly  zigzag  line  to  the  same  side. 
This  lateral  movement  was  not  caused  by  the  attachment  of 
the  glass  filaments,  nor  by  the  action  of  light ;  for  no  light  was 
allowed  to  enter  when  each  observation  was  made,  except  from 
vertically  above. 

*The  hypocotyl  of  a  seedling  was  secured  to  a  little  stick ;  it 
bore  nine  in  appearance  distinct  cotyledons,  arranged  in  a  circle. 
The  movements  of  two  nearly  opposite  ones  were  observed.  The 
tip  of  one  was  painted  white,  with  a  mark  placed  below,  and  the 
figure  described  (Fig.  44,  A)  shows  that  it  made  an  irregular 


Fig.  44. 


A.  B- 

finus  pinaster:  circumimtation  of  two  opposite  cotyledons,  traced  on 
horizontal  glass  in  darkness,  from  8.45  A.M.  to  8.35  P.M.  Nov.  25th. 
Movement  of  tip  in  A  magnified  about  22  times,  here  reduced  to  one- 
half  of  original  scale. 

circle  in  the  course  of  about  8  h.  During  the  night  it 
travelled  to  a  considerable  distance  in  the  direction  indicated 
by  the  broken  line.  A  glass  filament  was  attached  longitu- 
dinally to  the  other  cotyledon,  and  this  nearly  completed 
(Fig.  44,  B)  an  irregular  circular  figure  in  about  12  hours. 
During  the  night  it  also  moved  to  a  considerable  distance,  in 
the  direction  indicated  by  the  broken  line.  The  cotyledons 
therefore  circumnutate  independently  of  the  movement  of  the 
hypocotyl.  Although  they  moved  much  during  the  night,  they 
did  not  approach  each  other  so  as  to  stand  more  vertically  than 
during  the  day. 


58  CIRCUMNUTATION   OF   SEEDLINGS.         CHAP  I. 

Cycas-pectinaia  (CycadeaG). — The  large  seeds  of  this  plant  in 
germinating  first  protrude  a  single  leaf,  which  breaks  through 
the  ground  with  the  petiole  bowed  into  an  arch  and  with  the 
leaflets  involuted.  A  leaf  in  this  condition,  which  at  the  close 
of  our  observations  was  2 2  inches  in  height,  had  its  movements 
traced  in  a  warm  greenhouse  by  means  of  a  glass  filament 
bearing  paper  triangles  attached  across  its  tip.  The  tracing 
(Fig.  45)  shows  how  large,  complex,  and  rapid  were  the  circum- 

Fig.  45. 


Cycas  pectinata :  circumnutation  of  young  leaf  whilst  emerging  from  the 
ground,  feebly  illuminated  from  above,  traced  on  vertical  glass,  from 
5  P.M.  May  28th  to  11  A.M.-31st.  Movement  magnified  7  times,  here 
reduced  to  two-thirds  of  original  scale. 

nutating  movements.  The  extreme  distance  from  side  to  side 
which  it  passed  over  amounted  to  between  '6  and  '7  of  an 
inch. 

Canna  Warscewiczii  (Cannacese).— A  seedling  with  the  plu- 
mule projecting  one  inch  above  the  ground  was  observed,  but 
not  under  fair  conditions,  as  it  was  brought  out  of  the  hot- 
house and  kept  in  a  room  not  sufficiently  warm.  Nevertheless 
the  tracing  (Fig.  46)  shows  that  it  made  two  or  three  incom- 
plete irregular  circles  or  ellipses  in  the  course  of  48  hours.  The 
plumule  is  straight ;  and  this  was  the  first  instance  observed 


CHAP.  L 


ALLIUM. 


59 


by  us  of  the  part  that  first  breaks  through   the  ground  not 
being  arched. 

Fig  46. 


Canna  Warscevciczii :  circumnutation  of  plumule  with  filament  affixed 
obliquely  to  outer  sheath-like  leaf,  traced  in  darkness  onhorizontal  glass 
from  8.45  A.M.  Nov.  9th  to  8.10  A.M.  llth.  Movement  of  bead  mag- 
nified 6  times. 

Allium  cepa  (Liliacese).— The  narrow  green  leaf,  which  pro- 
trudes from  the  seed  of  the  common  onion  as  a  cotyledon,* 
breaks  through  the  ground  in  the  form  of  an  arch,  in  the  same 
manner  as  the  hypocotyl  or  epicotyl  of  a  dicotyledonous  plant. 
Long  after  the  arch  has  risen  above  the  surface  the  apex 
remains  within  the  seed-coats,  evidently  absorbing  the  still 
abundant  contents.  The  summit  or  crown  of  the  arch,  "when 
it  first  protrudes  from  the  seed  and  is  still  buried  beneath  the 
ground,  is  simply  rounded;  but  before  it  reaches  the  surface 
it  is  developed  into  a  conical  protuberance  of  a  white  colour 
(owing  to  the  absence  of  chlorophyll),  whilst  the  adjoining  parts 
are  green),  with  the  epidermis  apparently  rather  thicker  and 
tougher  than  elsewhere.  We  may  therefore  conclude  that  this 
conical  protuberance  is  a  special  adaptation  for  breaking  through 
the  ground,f  and  answers  the  same  end  as  the  knife-like  white 
crest  on  the  summit  of  the  straight  cotyledon  of  the  Graminese. 


*  This  is  the  expression  used 
by  Sachs  in  his  'Text-book  of 
Botany.' 

t  Haberlandt  has  briefly  de- 
scribed ('  Die  Schutzcinrichtun- 
gen  .  . .  Keimpflanze,'  1877,  p.  77) 
this  curious  structure  and  the 


purpose  which  it  subserves.  He 
states  that  good  figures  of  the 
cotyledon  of  the  onion  have  been 
given  by  Tittmann  and  by  Sachs 
in  his  'Experimental  Physiologie,' 
p.  93. 


60 


CIRCUMNUTATION   OF   SEEDLINGS. 


CHAP.  I. 


After  a  time  the  apex  is  drawn  out  of  the  empty  seed-coats, 
and  rises  up,  forming  a  right  angle,  or  more  commonly  a  still 
larger  angle  with  the  lower  part,  and  occasionally  the  whole 
becomes  nearly  straight.  The  conical  protuberance,  which 
originally  formed  the  crown  of  the  arch,  is  now  seated  on  one 
side,  and  appears  like  a  joint  or  knee,  which  from  acquiring 
chlorophyll  becomes  green,  and  increases  in  size.  In  rarely  or 
never  becoming  perfectly  straight,  these  cotyledons  differ  remark- 
ably from  the  ultimate  condition  of  the  arched  hypocotyls  or 
epicotyls  of  dicotyledons.  It  is,  also,  a  singular  circumstance 
that  the  attenuated  extremity  of  the  upper  bent  portion 
invariably  withers  and  dies. 

A  filament,  1'7  inch  in  length,  was  affixed  nearly  upright 
beneath  the  knee  to  the  basal  and  vertical  portion  of  a 
cotyledon;  and  its  movements  were 
traced  during  14  h.  in  the  usual  manner. 
The  tracing 'here  given  (Fig.  47)  indi- 
cates circumnutation.  The  movement  of 
the  upper  part  above  the  knee  of  the  same 
cotyledon,  which  projected  at  about  an 
angle  of  45°  above  the  horizon,  was 
observed  at  the  same  time.  A  filament 
was  not  affixed  to  it,  but  a  mark  was 
placed  beneath  the  apex,  which  was 
almost  white  from  beginning  to  wither, 
and  its  movements  were  thus  traced.  The 
Allium  cepa :  circumnu-  figure  described  resembled  pretty  closely 
tation  of  basal  half  that  above  given ;  and  this  shows  that  the 


Fig.  47. 


of  arched  cotyledon, 
traced  in  darkness  on 
horizontal  glass,  from 
8.15  A.M.  to  10  P.M. 
Oct.  31st.  Movement 
of  bead  magnified 
about  17  times. 


chief  seat  of  movement  is  in  the  lower  or 
basal  part  of  the  cotyledon. 

Asparagus  officinalis  (Asparagese). — 
The  tip  of  a  straight  plumule  or  cotyledon 
(for  we  do  not  know  which  it  should  be 
called)  was  found  at  a  depth  of  '1  inch 

beneath  the  surface,  and  the  earth  was  then  removed  all  round 
to  the  depth  of  -3  inch.  A  glass  filament  was  affixed  obliquely  to 
it,  and  the  movement  of  the  bead,  magnified  17  times,  was  traced 
in  darkness.  During  the  first  1  h.  15  m.  the  plumule  moved  to 
the  right,  and  during  the  next  two  hours  it  returned  in  a  roughly 
parallel  but  strongly  zigzag  course.  From  some  unknown  cause 
it  had  grown  up  through  the  soil  in  an  inclined  direction,  and 
now  through  apogeotropism  it  moved  during  nearly  '24  h.  in 


CHAP.  I. 


ASPARAGUS. 


61 


the  same  general  direction,  but  in  a  slightly  zigzag  manner, 
until  it  became  upright.  On  the  following  morning  it  changed 
its  course  completely.  There  can  therefore  hardly  be  a  doubt 
that  the  plumule  circumnutates,  whilst  buried  beneath  the 
ground,  as  much  as  the  pressure  of  the  surrounding  earth  will 
permit.  The  surface  of  the  soil  in  the  pot  was  now  covered  with 
a  thin  layer  of  very  fine  argillaceous  sand,  which  was  kept  damp; 
and  after  the  tapering  seedlings  had  grown  a  few  tenths  of 
an  inch  in  height,  each  was  found  surrounded  by  a  little  open 
space  or  circular  crack ;  and  this  could  be  accounted  for  only  by 
their  having  circumnutated  and  thus  pushed  away  the  sand  on 
all  sides ;  for  there  was  no  vestige  of  a  crack  in  any  other  part. 
In  order  to  prove  that  there  was  circumnutation,  the  move- 
Fig.  48. 


Asparagus  officinalis  :  circumnutation  of  plumules  with  tips  whitened  and 
marks  placed  beneath,  traced  on  a  horizontal  glass.  A,  young  plumule  ; 
movement  traced  from  8.30  A.M.  Nov.  30th  to  7.15  A.M.  next  morning  ; 
magnified  about  35  times.  B,  older  plumule  ;  movement  traced  from 
10.15  A.M.  to  8.10  P.M.  Nov.  29th  ;  magnified  9  times,  but  here  reduced 
to  one-half  of  original  scale. 

ments  of  five  seedlings,  varying  in  height  from  '3  inch  to  2  inches, 
were  traced.  They  were  placed  within  a  box  and  illuminated 
from  above;  but  in  all  five  cases  the  longer  axes  of  the  figures 
described  were  directed  to  nearly  the  same  point ;  so  that  more 
light  seemed  to  have  come  through  the  glass  roof  of  the  green- 
house on  one  side  than  on  any  other.  All  five  tracings  re- 
sembled each  other  to  a  certain  extent,  and  it  will  suffice  to  give 
two  of  them.  In  A  (Fig.  48)  the  seedling  was  only  45  of  an 


62  CIRCUMNUTATION  OF  SEEDLINGS.         CHAP.  I. 

inch  in  height,  and  consisted  of  a  single  internode  bearing  a 
bud  on  its  summit.  The  apex  described  between  8.30  A.M.  and 
10.20  P.M.  (i.e.  during  nearly  14  hours)  a  figure  which  would 
probably  have  consisted  of  3i  ellipses,  hai  not  the  stem  been 
drawn  to  one  side  until  1  P.M.,  after  which  hour  it  moved  back- 
wards. On  the  following  morning  it  was  not  far  distant  from 
the  point  whence  it  had  first  started.  The  actual  amount  of 
movement  of  the  apex  from  side  to  side  was  very  small,  viz. 
about  j^th  of  an  inch.  The  seedling  of  which  the  movements 
are  shown  in  Fig.  48,  B,  was  If  inch  in  height,  and  consisted  of 
three  internodes  besides  the  bud  on  the  summit.  The  figure, 
which  was  described  during  10  h.,  apparently  represents  two 
irregular  and  unequal  ellipses  or  circles.  The  actual  amount  of 
movement  of  the  apex,  in  the  line  not  influenced  by  the  light,  was 
•11  of  an  inch,  and  in  that  thus  influenced  '37  of  an  inch.  With 
a  seedling  2  inches  in  height  it  was  obvious,  even  without  the 
aid  of  any  tracing,  that  the  uppermost  part  of  the  stem  bent 
successively  to  all  points  of  the  compass,  like  the  stem  of  a 
twiaing  plant.  A  little  increase  in  the  power  of  circumnutating 
and  in  the  flexibility  of  the  stem,  would  convert  the  common 
asparagus  into  a  twining  plant,  as  has  occurred  with  one  species 
in  this  genus,  namely,  A.  scandens. 

Phalaris  Canariensis  (Gramineaj).  —  "With  the  Graminese  the 
part  which  first  rises  above  the  ground  has  been  called  by  some 
authors  the  pileole;  and  various  views  have  been  expressed  on 
its  homological  nature.  It  is  considered  by  some  great  authori- 
ties to  be  a  cotyledon,  which  term  we  will  use  without  venturing 
to  express  any  opinion  on  the  subject.*  It  consists  in  the 
present  case  of  a  slightly  flattened  reddish  sheath,  terminating 
upwards  in  a  sharp  white  edge;  it  encloses  a  true  green  leaf, 
whijh  protrudes  from  the  sheath  through  a  slit-like  orifice, 
close  beneath  and  at  right  angles  to  the  sharp  edge  on  the 
summit.  The  sheath  is  not  arched  when  it  breaks  through  the 
ground. 

The  movements  of  three  rather  old  seedlings,  about  Ik  inch 
in  height,  shortly  before  the  protrusion  of  the  leaves,  were  first 
traced.  They  were  illuminated  exclusively  from  above;  for,  as 
will  hereafter  be  shown,  they  are  excessively  sensitive  to  the 


*  We  are  indebted  to  the  Kev.       this  subject,   together    with    re- 
G.  Henslow  for  an  abstract  of  the       ferences. 
views  which  have  been  held  on 


CHAP.  I. 


PHALARIS. 


63 


Fig.  49. 


action  of  light ;  and  if  any  enters  even  temporarily  on  one  side, 

they  merely  bend  to  this  side  in  slightly  zigzag  lines.    Of  the  three 

tracings  one  alone  (Fig.  49)  is  here  given.    Had  the  observations 

been  more  frequent  during  the  12  h. 

two  oval  figures  would  have  been 

described  with  their  longer  axes  at 

right  angles  to  one  another.     The 

actual  amount  of  movement  of  the 

apex  from  side  to  side  was  about 

•3  of  an  inch.    The  figures  described 

by  the  other  two  seedlings  resembled 

to  a   certain  extent  the  one   here 

given. 

A  seedling  which  had  just  broken 
through'  the  ground  and  projected 
only  -^th  of  an  inch  above  the 
surface,  was  next  observed  in  the 
same  manner  as  before.  It  was 
necessary  to  clear  away  the  earth 
all  round  the  seedling  to  a  little 
depth  in  order  to  place  a  mark 

beneath  the  apex.  The  figure  (Fig.  50)  shows  that  the  apex 
moved  to  one  side,  but  changed  its  course  ten  times  in  the 
course  of  the  ten  hours  of  observa- 
tion ;  so  that  there  can  be  no  doubt 
about  its  circumnutation.  The 
cause  of  the  general  movement 
in  one  direction  could  hardly  be 
attributed  to  the  entrance  of 
lateral  light,  as  this  was  carefully 
guarded  against ;  and  we  suppose 
it  was  in  some  manner  connected 
with  the  removal  of  the  earth 
round  the  little  seedling. 

Lastly,  the  soil  in  the  same  pot 
was  searched  with  the  aid  of  a 
lens,  and  the  white  knife-like  apex 
of  a  seedling  was  found  on  an  exact 
level  with  that  of  the  surrounding 

surface.     The  soil  was  removed  all  round  the  apex  to  the  depth 
of  a  quarter  of  an  inch,  the  seed  itself  remaining  covered.    The 
pot,  protected  from  lateral  light,  was  placed  under  the  micro- 
4 


mark  placed  below  the  apex, 
traced  on  a  horizontal  glass, 
from  8.35  A.M.  Nov.  26th  to 
8.45A.M.  27th.  Movement  of 
apex  magnified  7  times,  here 
reduced  to  one-half  scale. 


Fig.  50. 


Phalaris  Canariensis :  circumnu- 
tation of  a  very  young  coty- 
ledon, with  a  mark  placed 
below  the  apex,  traced  on  a 
horizontal  glass,  from  11.37 
A.M.  to  9.30  P.M.  Dec.  13th. 
Movement  of  apex  greatly 
magnified,  here  reduced  to 
one-fourth  of  original  scale. 


CIRCUMNUTATION  OF   SEEDLINGS.        CHAP.  I. 


scope  with  a  micrometer  eye -piece,  so  arranged  that  each 
division  equalled  ^^th  of  an  inch.  After  an  interval  of  30.  m. 
the  apex  was  observed,  and  it  was  seen  to  cross  a  little  obliquely 
two  divisions  of  the  micrometer  in  9  m.  15  s. ;  and  after  a  few 
minutes  it  crossed  the  same  space  in  8  m.  50  s.  The  seedling 
was  again  observed  after  an  intervalof  three-quarters  of  an  hour, 
and  now  the  apex  crossed  rather  obliquely  two  divisions  in  10  m. 
We  may  therefore  conclude  that  it  was  travelling  at  about  the 
rate  of  -g^th  of  an  inch  in  45  minutes.  We  may  also  conclude 
from  these  and  the  previous  observations,  that  the  seedlings  of 
Phalaris  in  breaking  through  the  surface  of  the  soil  circum- 
nutate  as  much  as  the  surrounding  pressure  will  permit.  This 
fact  accounts  (as  in  the  case  before  given  of  the  asparagus)  for 
a  circular,  narrow,  open  space  or  crack  being  distinctly  visible 
round  several  seedlings  which  had  risen  through  very  fine 
argillaceous  sand,  kept  uniformly  damp. 

Zea  mays  (Grammes). — A  glass  filament  was  fixed  obliquely 

to  the  summit  of  a  cotyledon, 
rising  -2  of  an  inch  above  the 
ground ;  but  by  the  third  morn- 
ing it  had  grown  to  exactly 
thrice  this  height,  so  that  the 
distance  of  the  bead  from  the 
mark  below  was  greatly  in- 
creased, consequently  the  trac- 
ing (Fig.  51)  was  much  more 
magnified  on  the  first  than  on 
the  second  day.  The  upper 
part  of  the  cotyledon  changed 
^_ > -  its  course  by  at  least  as  much 


Fig.  51. 


Zea  mays :  circumnutation  of  cotyle- 
don, traced  on  horizontal  glass,  from 
8.30  A.M.  Feb.  4th  to  8  A.M.  6th. 
Movement  of  bead  magnified  on  an 
average  about  25  times. 


as  a  rectangle  six  times  on  each 
of  the  two  days.  The  plant 
was  illuminated  by  an  obscure 
light  from  vertically  above. 
This  was  a  necessary  precau- 
tion, as  on  the  previous  day  we  had  traced  the  movements  of 
cotyledons  placed  in  a  deep  box,  the  inner  side  of  which  was 
feebly  illuminated  on  one  side  from  a  distant  north-east  window, 
and  at  each  observation  by  a  wax  taper  held  for  a  minute  or 
two  on  the  same  side ;  and  the  result  was  that  the  cotyledons 
travelled  all  day  long  to  this  side,  though  making  in  their  course 
some  conspicuous  flexures,  from  which  fact  alone  we  might  have 


CHAP.  I.  PHALAEIS.  65 

concluded  that  they  were  circumnutating ;  but  we  thought  it 
advisable  to  make  the  tracing  above  given. 

Radicles. — Glass  filaments  were  fixed  to  two  short  radicles, 
placed  so  as  to  stand  almost  upright,  and  whilst  bending  down- 
wards through  geotropism  their  courses  were  strongly  zigzag ; 
from  this  latter  circumstance  circumnutation  might  have  been 
inferred,  had  not  their  tips  become  slightly  withered  after  the 
first  24  h.,  though  they  were  watered  and  the  air  kept  very 
damp.  Nine  radicles  were  next  arranged  in  the  manner 
formerly  described,  so  that  in  growing  downwards  they  left 
tracks  on  smoked  glass-plates,  inclined  at  various  angles  between 
45°  and  80°  beneath  the  horizon.  Almost  every  one  of  these 
tracks  offered  evidence  in  their  greater  or  less  breadth  in  dif- 
ferent parts,  or  in  little  bridges  of  soot  being 
left,  that  the  apex  had  come  alternately  into  Fig.  52. 
more  and  less  close  contact  with  the  glass.  In 
the  accompanying  figure  (Fig.  52)  we  have 
an  accurate  copy  of  one  such  track.  In  two 
instances  alone  (and  in  these  the  plates  were 
highly  inclined)  there  was  some  evidence  of 
slight  lateral  movement.  We  presume  therefore 
that  the  friction  of  the  apex  on  the  smoked 
surface,  little  as  this  could  have  been,  sufficed 
to  check  the  movement  from  side  to  side  of  these  ™ 

, .  .  Zea  mays :  track 

delicate  radicles.  left  on  incliQed 

A  vena  saliva  (Graminese). — A  cotyledon,  1£  smoked  .glass- 
inch  in  height,  was  placed  in  front  'of  a  north-  PJ.ate  ^  *!P 
east  window,  and  the  movement  of  the  apex  growTngdown- 
was  traced  on  a  horizontal  glass  during  two  wards, 
days.  It  moved  towards  the  light  in  a  slightly 
zigzag  line  from  9  to  11.30  A.M.  on  October  15th ;  it  then  moved 
a  little  backwards  and  zigzagged  much  until  5  P.M.,  after  which 
hour,  and  during  the  night,  it  continued  to  move  towards  the 
window.  On  the  following  morning  the  same  movement  was 
continued  in  a  nearly  straight  line  until  12.40  P.M.,  when  the  sky 
remained  until  2.35  extraordinarily  dark  from  thunder-clouds. 
During  this  interval  of  1  h.  55  m.,  whilst  the  light  was  obscure, 
it  was  interesting  to  observe  how  circumnutation  overcame 
heliotropism,  for  the  apex,  instead  of  continuing  to  move  towards 
the  window  in  a  slightly  zigzag  line,  reversed  its  course  four 
times,  making  two  small  narrow  ellipses.  A  diagram  of  this  case 
will  be  given  in  the  chapter  on  Heliotropism. 


66 


CIRCUMNUTATION  OF   SEEDLINGS.         CHAP.  I. 


A  filament  was  next  fixed  to  a  cotyledon  only  £  of  an  inch  in 
height,  which  was  illuminated  exclusively  from  above,  and  as 
it  was  kept  in  a  warm  greenhouse,  it  grew  rapidly ;  and  now 
there  could  be  no  doubt  about  its  circumnutation,  for  it  described 
a  figure  of  8  as  well  as  two  small  ellipses  in  5  2  hours. 

Nephrodium  molle  (Filices). — A  seedling  fern  of  this  species 


Fig.  53. 


Nephrodium  molle :  circumnutation 
of  very  young  frond,  traced  in 
darkness  on  horizontal  glass, 
from  9  A  M.  to  9.  P.M.  Oct.  30th. 
Movement  of  bead  magnified  48 
times. 


came  up  by  chance  in  a  flower- 
pot near  its  parent.  The  frond, 
as  yet  only  slightly  lobed,  was 
only  "16  of  an  inch  in  length  and 
•2  in  breadth,  and  was  supported 
on  a  rachis  as  fine  as  a  hair 
and  '23  of  an  inch  in  height.  A 
very  thin  glass  filament,  which 
projected  for  a  length  of  -36  of 
an  inch,  was  fixed  to  the  end  of 
the  frond.  The  movement  was 
so  highly  magnified  that  the 
figure  (Fig.  53)  cannot  be  fully 
trusted;  but  the  frond  was 
constantly  moving  in  a  complex 
manner,  and  the  bead  greatly 


Fig.  54. 


changed  its  course  eighteen  times  in  the  12  hours  of  observation. 
Within  half  an  hour  it  often  returned  in  a  line  almost  parallel 
to  its  former  course.    The  greatest  amount  of  movement  occurred 
between  4  and  6  P.M.     The-  circumnuta- 
tion of  this  plant  is  interesting,  because 
the  species  in  the  genus  Lygodium  are 
well  known  to  circumnutate  conspicuously 
and  to   twine  round  any  neighbouring 
object. 

Selaginella  Kraussii  (?)  (Lycopodiacerc). 

circumnutation  of  — A  very  young  plant,  only  '4  of  an  inch 
young  plant,  kept  in  in  height,  had  sprung  up  in  a  pot  in  the 
hot-house.  An  extremely  fine  glass  fila- 
ment was.  fixed  to  the  end  of  the  frond- 
like  stem,  and  the  movement  of  the  bead 
traced  on  a  horizontal  glass.  It  changed  its  course  several 
times,  as  shown  in  Fig.  54,  whilst  observed  during  13  h.  15  m., 
and  returned  at  night  to  a  point  not  far  distant  from  that 
whence  it  had  started  in  the  morning.  There  can  be  no  doubt 
that  this  little  plant  circumnutated. 


Selaginella  Kraussii  (?) : 


darkness,  traced  from 
8.45  A.M.  to  10  P.M. 
Oct.  31st. 


CHAP.  II.        CIBCUMtfUTATION  OF  SEEDLINGS.  t>7 


CHAPTEE  II. 

GENERAL  CONSIDERATIONS  ON  THE  MOVEMENTS  AND  GROWTH  OP 
SEEDLING  PLANTS. 

Generality  of  the  circumnutating  movement — Kadicles,  their  circum- 
nutation  of  service — Manner  in  which  they  penetrate  the  ground — 
Manner  in  which  hypocotyls  and  other  organs  break  through  the 
ground  by  being  arched — Singular  manner  of  germination  in  Megar. 
rlnza,  &c. — Abortion  of  cotyledons—  Circumnutation  of  hypocotyls 
and  epicotyls  whilst  still  buried  and  arched — Their  power  of 
straightening  themselves — Bursting  of  the  seed-coats—inherited 
effect  of  the  arching  process  in  hypogean  hypocotyls— Circumnuta- 
tion of  hypocotyls  and  epicotyls  when  erect — Ciicumnutation  of 
cotyledons — Pulvini  or  joints  of  cotyledons,  duration  of  their 
activity,  rudimentary  in  Oxalis  corniculata,  their  development — 
Sensitiveness  of  cotyledons  to  light  and  consequent  disturbance  of 
their  periodic  movements — Sensitiveness  of  cotyledons  to  contact. 

THE  circumnutating  movements  of  the  several  parts 
or  organs  of  a  considerable  number  of  seedling  plants 
have  been  described  in  the  last  chapter.  A  list  isjhere 
appended  of  the  Families,  Cohorts,  Sub-classes,  &c., 
to  which  they  belong,  arranged  and  numbered  ac- 
cording to  the  classification  adopted  by  Hooker.* 
Any  one  who  will  consider  this  list  will  see  that  the 
young  plants  selected  for  observation,  fairly  represent 
the  whole  vegetable  series  excepting  the  lowest 
cryptogams,  and  the  movements  of  some  of  the  latter 
when  mature  will  hereafter  be  described.  As  all  the 
seedlings  which  were  observed,  including  Conifers, 
Cycads  and  Ferns,  which  belong  to  the  most  ancient 


*  As  given  in  the   '  General  System  of  Botany,'  by  Le  Maout  and 
Decaisne,  1873. 


68  CIKCUMNUTATION   OF  SEEDLINGS.        CHAP.  II. 

types  amongst  plants,  were  continually  circumnu- 
tating,  we  may  infer  that  this  kind  of  movement  is 
common  to  every  seedling  species. 

SUB-KINGDOM  I — Phaenogamous  Plants. 
Class  I.— DICOTYLEDONS. 
Sub-class  I  — Angiosperms. 

Family.  Cohort. 

14.  Cntciferce.  II.  PARIETALES. 

26.  Caryopkyllece.  IV.  CARYOPHYLLALES. 

36    Malvaceae.  VI.  MALVALES. 

41.  Oxalidece.  VII.  GERANIALES. 

49.  Tropceolece.  DITTO 

52.  Aurantiacece.  DITTO 

70.  HippocastanecB.  X.  SAPINDALES. 

75.  Leguminosce.  XI.  ROSALES. 

106.  CvcurbitucecB.  XII.  PASSIFLORALES. 

109.  Cactece.  XIV.  FICOIDALES. 

122.  Composites.  XVII.  ASTRALES. 

135.  Primulacece.  XX.  PRIMULALES. 

145.  Aselepiadece.  XXII.  GENTIANALES. 

151.  Convolvulaceos.  XXIII.  POLEMONIALES. 

154.  BorraginecB.  DITTO 

156.  Nolanecs.  DITTO 

157.  Solanece.  XXIV.  SOLANALES. 
181.  Chenopodiece.  XXVII.  CHENOPODIALES. 
202.  Euphorbiacece.  XXXII.  EUPHORBIA LES. 

211.  Cupuliferce.  XXXVI.  QUERNALES. 

212.  Corylacea.  DITTO 

Sub-class  II. — Gymnosperms. 

223.  Conifera. 

224.  CycadecB. 

Class  II. — MONOCOTYLEDONS. 

2.  Cannacece.  II.  AMOMALES. 

34.  Liliacece.  XL  LILIALES. 

41.  Asparagece.  DITTO 

55.  Graminecs.  XV.  GLUM  ALES. 

SUB-KINGDOM  II. — Cryptogamic  Plants. 
1.  Filices.  I.  FILICALES. 

6.  Lycopodiaceas.  DITTO 


CIIAP.  II.  ACTION  OP   THE  EADICLE.  69 

Radicles. — In   all   the  germinating  seeds  observed 
by  us,   the   first   change    is    the    protrusion   of   the 
radicle,   which    immediately   bends    downwards    and 
endeavours   to   penetrate    the   ground.     In   order  to 
effect  this,  it  is  almost  necessary  that  the  seed  should 
be  pressed  down  so  as  to  offer  some  resistance,  unless 
indeed  the  soil  is  extremely  loose ;  for  otherwise  the 
seed  is  lifted  up,  instead  of  the  radicle  penetrating 
the  surface.     But  seeds  often  get  covered  by  earth 
thrown   up  by  burrowing   quadrupeds   or   scratching 
birds,  by  the   castings  of  earth-worms,  by  heaps   of 
excrement,  the  decaying  branches  of  trees,  &c.,  and 
will  thus  be  pressed  down ;  and  they  must  often  fall 
into  cracks  when  the  ground  is  dry,  or  into  holes. 
Even  with  seeds  lying  on  the  bare  surface,  the  first 
developed  root-hairs,  by  becoming  attached  to  stones 
or  other  objects  on  the  surface,  are  able  to  hold  down 
the  upper   part  of  the   radicle,  whilst  the  tip  pene- 
trates the  ground.     Sachs  has  shown*  how  well  and 
closely  root-hairs  adapt  themselves  by  growth  to  the 
most  irregular  particles  in  the  soil,  and  become  firmly 
attached   to   them.      This    attachment   seems   to   be 
effected  by  the  softening  or  liquefaction  of  the-outer 
surface  of  the  wall  of  the   hair  and  its   subsequent 
consolidation,    as    will    be   on   some   future   occasion 
more  fully  described.     This  intimate  union  plays  an 
important  part,  according  to  Sachs,  in  the  absorption 
of  water  and  of  the  inorganic  matter  dissolved  in  it. 
The  mechanical  aid  afforded  by  the  root-hairs  in  pene- 
trating   the    ground   is   probably   only   a    secondary 
service. 

The  tip  of  the  radicle,  as  soon  as  it  protrudes  from 
the  seed-coats,  begins  to  circumnutate,  and  the  whole 


'Physiologie  Vegetale,'  18G8,  pp.  199,  205. 


70  ACTION   OF   THE   EADICLE.  CHAP.  IL 

growing  part  continues  to  do  so,  probably  for  as  long 
as  growth  continues.  This  movement  of  the  radicle 
has  been  described  in  Brassica,  ^Esculus,  Phaseolus, 
Vicia,  Cucurbita,  Quercus  and  Zea.  The  probability 
of  its  occurrence  was  inferred  by  Sachs,*  from  radicles 
placed  vertically  upwards  being  acted  on  by  geotro- 
pism  (which  we  likewise  found  to  be  the  case),  for  if 
they  had  remained  absolutely  perpendicular,  the  attrac- 
tion of  gravity  could  not  have  caused  them  to  bend  to 
any  one  side.  Circuinnutation  was  observed  in  the  above 
specified  cases,  either  by  means  of  extremely  fine  fila- 
ments of  glass  affixed  to  the  radicles  in  the  manner 
previously  described,  or  by  their  being  allowed  to 
grow  downwards  over  inclined  smoked  glass-plates,  on 
which  they  left  their  tracks.  In  the  latter  cases  the 
serpentine  course  (see  Figs.  19,  21,  27,  41)  showed 
unequivocally  that  the  apex  had  continually  moved 
from  side  to  side.  This  lateral  movement  was  small 
in  extent,  being  in  the  case  of  Phaseolus  at  most 
about  1  mm.  from  a  medial  line  to  both  sides.  But 
there  was  also  movement  in  a  vertical  plane  at  right 
angles  to  the  inclined  glass-plates.  This 'was  shown 
by  the  tracks  often  being  alternately  a  little  broader 
and  narrower,  due  to  the  radicles  having  alternately 
pressed  with  greater  and  less  force  on  the  plates. 
Occasionally  little  bridges  of  soot  were  left  across  the 
tracks,  showing  that  the  apex  had  at  these  spots  been 
lifted  up.  This  latter  fact  was  especially  apt  to  occur 


*  'Ueber  das  Wachsthum  der  had  previously  remarked  (*  Bti- 

"Wurzeln :  Arbeiten  des  bot.  In-  trage    zur    Pflanzenphysiologie,' 

Btituts    in   Wiirzburg,'    Heft   iii.  18G8,  p.  81)  on  the  fact  of  radicles 

1873,  p.  460.     This  memoir,  be-  placed   vertically  upwards  being 

sides  its   intrinsic  and   great  in-  acted  on  by  geotropism,  and   he 

terest,  deserves  to  be  studied  as  a  explained  it   by  the   supposition 

model    of   careful    investigation,  that  their  growth  was  not  equal 

and    we   shall   have   occasion   to  on  all  sides, 
refer  to  it  repeatedly.     Dr.  Frank 


CHAP.  II.  ACTION  OF  THE  BADICLE.  71 

when  the  radicle  instead  of  travelling  straight  down 
the  glass  made  a  semicircular  bend;  but  Fig.  52 
shows  that  this  may  occur  when  the  track  is  rectilinear. 
The  apex  by  thus  rising,  was  in  one  instance  able  to 
surmount  a  bristle  cemented  across  an  inclined  glass- 
plate  ;  but  slips  of  wood  only  ^  of  an  inch  in  thickness 
always  caused  the  radicles  to  bend  rectangularly  to 
one  side,  so  that  the  apex  did  not  rise  to  this  small 
height  in  opposition  to  geotropism. 

In  those  cases  in  which  radicles  with  attached  fila- 
ments were  placed  so  as  to  stand  up  almost  vertically, 
they  curved  downwards  through  the  action  of  geotro- 
pism, circumnutating  at  the  same  time,  and  their 
courses  were  consequently  zigzag.  Sometimes,  how- 
ever, they  made  great  circular  sweeps,  the  lines  being 
likewise  zigzag. 

Radicles  closely  surrounded  by  earth,  even  when 
this  is  thoroughly  soaked  and  softened,  may  perhaps 
be  quite  prevented  from  circumnutating.  Yet  we 
should  remember  that  the  circumnutating  sheath-like 
cotyledons  of  Phalaris,  the  hypocotyls  of  Solanum, 
and  the  epicotyls  of  Asparagus  formed  round  them- 
selves little  circular  cracks  or  furrows  in  a  superficial 
layer  of  damp  argillaceous  sand.  They  were  also 
able,  as  well  as  the  hypocotyls  of  Brassica,  to  form 
straight  furrows  in  damp  sand,  whilst  circumnutating 
and  bending  towards  a  lateral  light.  In  a  future 
chapter  it  will  be  shown  that  the  rocking  or  circum- 
nutating movement  of  the  flower-heads  of  TrifoUum 
subterraneum  aids  them  in  burying  themselves.  It  is 
therefore  probable  that  the  circumnutation  of  the  tip 
of  the  radicle  aids  it  slightly  in  penetrating  the 
ground ;  and  it  may  be  observed  in  several  of  the 
previously  given  diagrams,  that  the  movement  is 
more  strongly  pronounced  in  radicles  when  they  first 


72  ACTION  OF  THE  RADICLE.  CHAP.  H. 

protrude  from  the  seed  than  at  a  rather  later  period ; 
but  whether  this  is  an  accidental  or  an  adaptive 
coincidence  we  do  not  pretend  to  decide.  Never- 
theless, when  young  radicles  of  Phaseolus  multiflorus 
were  fixed  vertically  close  over  damp  sand,  in  the 
expectation  that  as  soon  as  they  reached  it  they 
would  form  circular  furrows,  this  did  not  occur, — a 
fact  which  may  be  accounted  for,  as  we  believe,  by 
the  furrow  being  filled  up  as  soon  as  formed  by  the 
rapid  increase  of  thickness  in  the  apex  of  the  radicle. 
Whether  or  not  a  radicle,  when  surrounded  by  soft- 
ened earth,  is  aided  in  forming  a  passage  for  itself 
by  circumnutating,  this  movement  can  hardly  fail 
to  be  of  high  importance,  by  guiding  the  radicle 
along  a  line  of  least  resistance,  as  will  be  seen  in  the 
next  chapter  when  we  treat  of  the  sensibility  of  the 
tip  to  contact.  If,  however,  a  radicle  in  its  down- 
ward growth  breaks  obliquely  into  any  crevice,  or  a 
hole  left  by  a  decayed  root,  or  one  made  by  the 
larva  of  an  insect,  and  more  especially  by  worms,  the 
circumnutating  movement  of  the  tip  will  materially 
aid  it  in  following  such  open  passage ;  and  we  have 
observed  that  roots  commonly  run  down  the  old 
burrows  of  worms.* 

When  a  radicle  is  placed  in  a  horizontal  or  inclined 
position,  the  terminal  growing  part,  as  is  well  known, 
bends  down  towards  the  centre  of  the  earth ;  and 
Sachs  f  has  shown  that  whilst  thus  bending,  the  growth 
of  the  lower  surface  is  greatly  retarded,  whilst  that 


*  See,  also,  Prof.  Hensen's  state-  rovvs  made  by  worms, 
merits  ('  Zeitschrift    fiir   Wissen,  f  '  Arbeiten    des    bot.      Inst. 

Zool.,'  B.  xxviii.  p.  354,  1877)  to  Wiirzburg,'  vol.  i.  1873,  p.  461. 

the  same  effect.     He  goes  so  far  See  also  p.  397  for  the  length  of 

as  to  believe  that  roots  are  able  the  growing  part,  and  p.  451  on 

to  penetrate  the  ground  to  a  great  the  force  of  geotropi&m. 
depth  only  by  means  of  the  bur- 


CHAI-.  II.  ACTION  OF  THE  RADICLE.  73 

of  the  upper  surface  continues  at  the  normal  rate, 
or  may  be  even  somewhat  increased.  He  has  further 
shown  by  attaching  a  thread,  running  over  a  pulley, 
to  a  horizontal  radicle  of  large  size,  namely,  that 
of  the  common  bean,  that  it  was  able  to  pull  up  a 
weight  of  only  one  gramme,  or  15*4  grains.  We  may 
therefore  conclude  that  geotropism  does  not  give  a 
radicle  force  sufficient  to  penetrate  the  ground,  but 
merely  tells  it  (if  such  an  expression  may  be  used) 
which  course  to  pursue.  Before  we  knew  of  Sachs' 
more  precise  observations  we  covered  a  flat  surface  of 
damp  sand  with  the  thinnest  tin-foil  which  we  could 
procure  ('02  to  '03  mm.,  or  -00012  to  -00079  of  an  inch 
in  thickness),  and  placed  a  radicle  close  above,  in  such 
a  position  that  it  grew  almost  perpendicularly  down- 
wards. When  the  apex  came  into  contact  with  the 
polished  level  surface  it  turned  at  right  angles  and 
glided  over  it  without  leaving  any  impression;  yet 
the  tin-foil  was  so  flexible,  that  a  little  stick  of  soft 
wood,  pointed  to  the  same  degree  as  the  end  of  the 
radicle  and  gently  loaded  with  a  weight  of  only  a 
quarter  of  an  ounce  (120  grains)  plainly  indented  the 
tin-foil. 

Eadicles  are  able  to  penetrate  the  ground  by  the 
force  due  to  their  longitudinal  and  transverse  growth ; 
the  seeds  themselves  being  held  down  by  the  weight 
of  the  superincumbent  soil.  In  the  case  of  the  bean 
the  apex,  protected  by  the  root-cap,  is  sharp,  and 
the  growing  part,  from  8  ,to  10  mm.  in  length,  is 
much  more  rigid,  as  Sachs  has  proved,  than  the  part 
immediately  above,  which  has  ceased  to  increase  in 
length.  We  endeavoured  to  ascertain  the  downward 
pressure  of  the  growing  part,  by  placing  germinating 
beans  between  two  small  metal  plates,  the  upper  one 
of  which  was  loaded  with  a  known  weight;  and  the 


74  ACTION  OF  THE  RADICLE.  CHAP.  II. 

radicle  was  then  allowed  to  grow  into  a  narrow  hole  in 
wood,  2  or  3  tenths  of  an  inch  in  depth,  and  closed  at 
the  bottom.  The  wood  was  so  cut  that  the  short  space 
of  radicle  between  the  mouth  of  the  hole  and  the 
bean  could  not  bend  laterally  on  three  sides ;  but  it 
was  impossible  to  protect  the  fourth  side,  close  to 
the  bean.  Consequently,  as  long  as  the  radicle  con- 
tinued to  increase  in  length  and  remained  straight, 
the  weighted  bean  would  be  lifted  up  after  the  tip 
had  reached  the  bottom  of  the  shallow  hole.  Beans 
thus  arranged,  surrounded  by  damp  sand,  lifted  up  a 
quarter  of  a  pound  in  24  h.  after  the  tip  of  the 
radicle  had  entered  the  hole.  With  a  greater  weight 
the  radicles  themselves  always  became  bent  on  the  one 
unguarded  side;  but  this  probably  would  not  have 
occurred  if  they  had  been  closely  surrounded  on  all 
sides  by  compact  earth.  There  was,  however,  a 
possible,  but  not  probable,  source  of  error  in  these 
trials,  for  it  was  not  ascertained  whether  the  beans 
themselves  go  on  swelling  for  several  days  after  they 
have  germinated,  and  after  having  been  treated  in 
the  manner  in  which  ours  had  been ; 
__^^  namely,  being  first  left  for  24  h.  in 
water,  then  allowed  to  germinate  in 
very  damp  air,  afterwards  placed  over 
Outline  of  piece  of  the  hole  and  almost  surrounded  by 

fn^a^'naturS    <^mp  sand  in  a  closed  box. 

size)  with  a  hole        We  succeeded  better  in  ascertaining 

the°Ur?dicie  WofiCa    ^  force  exerted  transversely  by  these 

bean  grew.  Thick-   radicles.     Two   were   so   placed  as  to 

narrow  end°  -as    penetrate  small  holes  made  in  little 

inch,  at  broad  end    sticks,  one  of  which  was  cut  into  the 

hole'  •  i  inch,          shape  here   exactly  copied  (Fig.  55). 

The   short   end   of  the  stick   beyond 

the  hole  was   purposely  split,  but   not   the   opposite 


GUAP.  II. 


ACTION   OF  THE   EADICLE. 


75 


Fig.  56. 


end.  As  the  wood  was  highly  elastic,  the  split  or 
fissure  closed  immediately  after  being  made.  After 
six  days  the  stick  and  bean  were  dug  out  of  the  damp 
sand,  and  the  radicle  was  found  to  be  much  enlarged 
above  and  beneath  the  hole.  The  fissure,  which  was 
at  first  quite  closed,  was  now  open  to  a  width  of 
4  mm. ;  as  soon  as  the  radicle  was  extracted,  it  imme- 
diately closed  to  a  width  of  2  mm.  The  stick  was 
then  suspended  horizontally  by 
a  fine  wire  passing  through  the 
hole  lately  filled  by  the  radicle, 
and  a  little  saucer  was  sus- 
pended beneath  to  receive  the 
weights ;  and  it  required  8  Ibs. 
8  ozs.  to  open  the  fissure  to  the 
width  of  4  mm. — that  is,  the 
width  before  the  root  was  ex- 
tracted. But  the  part  of  the 
radicle  (only  '1  of  an  inch  in 
length)  which  was  embedded  in 
the  hole,  probably  exerted  a 
greater  transverse  strain  even 
than  8  Ibs.  8  ozs.,  for  it  had  split 
the  solid  wood  for  a  length  of 
rather  more  than  a  quarter  of 
an  inch  (exactly  -275  inch),  and 

this  fissure  is  shown  in  Fig.  55.    Wo°den  pincers,  kept  closed  by 

A  second  stick  was  tried  in  the 
same  manner  with  almost  ex- 
actly the  same  result. 

We  then  followed  a  better 
plan.  Holes  were  bored  near 
the  narrow  end  of  two  wooden  clips  or  pincers  (Fig.  56), 
kept  closed  by  brass  spiral  springs.  Two  radicles  in  damp 
sand  were  allowed  to  grow  through  these  holes.  The 


spiral  brass  spring,  with  a 
hole  (*14  inch  in  diameter 
and  -6  inch  in  depth)  bored 
through  the  narrow  closed 
part,  through  which  a  radicle 
of  a  bean  was  allowed  to 
grow.  Temp.  50°-60°  F. 


76  ACTION  OF  THE  EADICLE.  CHAP.  II. 

pincers  rested  on  glass-plates  to  lessen  the  friction  from 
the  sand.  The  holes  were  a  little  larger  (viz.  -14  inch) 
and  considerably  deeper  (viz.  -6  inch)  than  in  the 
trials  with  the  sticks ;  so  that  a  greater  length  of  a 
rather  thicker  radicle  exerted  a  transverse  strain. 
After  13  days  they  were  taken  up.  The  distance  of 
two  dots  (see  the  figure)  on  the  longer  ends  of  the 
pincers  was  now  carefully  measured ;  the  radicles  were 
then  extracted  from  the  holes,  and  the  pincers  of 
course  closed.  They  were  then  suspended  horizontally 
in  the  same  manner  as  were  the  bits  of  sticks,  and  a 
weight  of  1500  grams  (or  3  Ibs.  4  ozs.)  was  necessary 
with  one  of  the  pincers  to  open  them  to  the  same 
extent  as  had  been  effected  by  the  transverse  growth 
of  the  radicle.  As  soon  as  this  radicle  had  slightly 
opened  the  pincers,  it  had  grown  into  a  flattened  form 
and  had  escaped  a  little  beyond  the  hole ;  its  diameter 
in  one  direction  being  4'2  mm.,  and  at  right  angles 
3'5  mm.  If  this  escape  and  flattening  could  have 
been  prevented,  the  radicle  would  probably  have 
exerted  a  greater  strain  than  the  3  Ibs.  4  ozs.  With 
the  other  pincers  the  radicle  escaped  still  further 
out  of  the  hole;  and  the  weight  required  to  open 
them  to  the  same  extent  as  had  been  effected  by  the 
radicle,  was  only  600  grams. 

With  these  facts  before  us,  there  seems  little  diffi- 
culty in  understanding  how  a  radicle  penetrates  the 
ground.  The  apex  is  pointed  and  is  protected  by 
the  root-cap ;  the  terminal  growing  part  is  rigid,  and 
increases  in  length  with  a  force  equal,  as  far  as  our 
observations  can  be  trusted,  to  the  pressure  of  at  least 
a  quarter  of  a  pound,  probably  with  a  much  greater 
force  when  prevented  from  bending  to  any  side  by  the 
surrounding  earth.  Whilst  thus  increasing  in  length 
it  increases  in  thickness,  pushing  away  the  damp 


CHAP.  II.          HYPOCOTYLS  AND  EPICOTYLS.  77 

earth  on  all  sides,  with  a  force  of  above  8  pounds  in 
one  case,  of  3  pounds  in  another  case.  -  It  was  impos- 
sible to  decide  whether  the  actual  apex  exerts,  relatively 
to  its  diameter,  the  same  transverse  strain  as  the  parts 
a  little  higher  up  ;  but  there  seems  no  reason  to  doubt 
that  this  would  be  the  case.  The  growing  part  there- 
fore does  not  act  like  a  nail  when  hammered  into  a 
board,  but  more  like  a  wedge  of  wood,  which  whilst 
slowly  driven  into  a  crevice  continually  expands  at 
the  same  time  by  the  absorption  of  water;  and  a 
wedge  thus  acting  will  split  -even  a  mass  of  rock. 

Manner  in  which  Hypocotijls,  Epicotyls,  <£<?.,  rise  up 
and  break  through  the  ground. — After  the  radicle  has 
penetrated  the  ground  and  fixed  the  seed,  the  hypo- 
cotyls  of  all  the  dicotyledonous  seedlings  observed  by 
us,  which  lift  their  cotyledons  above  the  surface,  break 
through  the  ground  in  the  form  of  an  arch.  When 
the  cotyledons  are  hypogean,  that  is,  remain  buried  in 
the  soil,  the  hypocotyl  is  hardly  developed,  and  the 
epicotyl  or  plumule  rises  in  like  manner  as  an  arch 
through  the  ground.  In  all,  or  at  least  in  most  of  such 
cases,  the  downwardly  bent  apex  remains  for  a  time 
enclosed  within  the  seed-coats.  With  Corylus  -avel- 
lena  the  cotyledons  are  hypogean,  and  the  epicotyl 
is  arched;  but  in  the  particular  case  described  in 
the  last  chapter  its  apex  had  been  injured,  and  it 
grew  laterally  through  the  soil  like  a  root;  and  in 
consequence  of  this  it  had  emitted  two  secondary 
shoots,  which  likewise  broke  through  the  ground  as 
arches. 

Cyclamen  does  not  produce  any  distinct  stem,  and 
only  a  single  cotyledon  appears  at  first ;  *  its  petiole 


*  This  is  the  conclusion  arrived  considered  by  other  botanists  as 

at    by   Dr.   H.   Gressner   ('Bot.  the   first  true  leaf  is  really  the 

Zeitung,'    1874,    p.    837),    who  second  cotyledon,  which  is  greatly 

maintains    that   what    has   been  delayed  in  its  development. 


78 


HYPOCOTYLS,  EPICOTYLS,  ETC.,        CHAP.  IL 


breaks   through   the    ground   as   an   arch   (Fig.   57). 


Fig.  57. 


Cyclamen 


Abronia  also  has  only  a  single  fully 
developed  cotyledon,  but  in  this 
case  it  is  the  hypocotyl  which  first 
emerges  and  is  arched.  Abronia 
umbellata,  however,  presents  this 
peculiarity,  that  the  enfolded  blade 
of  the  one  developed  cotyledon 
(with  the  enclosed  endosperm) 
whilst  still  beneath  the  surface  has 


straighten  itself;  A, 

hiTaOTrm  ;  r,Iecond- 
ary  radicles. 


1?'  KQ 


Persicum  : 

cfTdeTf    its  apex  upturned  and  parallel  to 
cotyledon,   not    yet    the  descending  leg   of  the  arched 
hypocotyl  ;     but    it     is    dragged 
O11t    of   the    ground    by  the   con- 
tinned    growth   of   the   hypocotyl, 
with  the  apex  pointing  downward. 
With  Cyeas  pectinata  the  cotyledons  are  hypogean, 

and  a  true  leaf  first  breaks 
through  the  ground  with 
its  petiole  forming  an 
arch. 

In  the  genus  Acanthus 
the  cotyledons  are  likewise 
hypogean.  In  A.  mollis, 
a  single  leaf  first  breaks 
through  the  ground  with 
its  petiole  arched,  and  with 
the  opposite  leaf  much  less 
developed,  short,  straight, 
of  a  yellowish  colour,  and 

Acanthus  moUis:  seedling,  with  the     with  the  petiole  at  first  not 

^^d^W-1^    half  as  thick  as  that  of  the 

off:  a,  blade  of  first  leaf  begin-     otner.        The      Undeveloped 

°S:Ti£t-"    leaf  is  protected  by  stand- 

opposite  leaf,  as  yet  very  imper-     -nff    beneath  its  arched  fel- 

fectly  developed;    c,  hypogean          s  . 

cotyledon  on  the  opposite  side.        low  J   and  it   IS   an    lUStrUC- 


CHAP.  II.        BREAKING  THEOUGH  THE  GROUND.          79 

tive  fact  that  it  is  not  arched,  as  it  has  not  to  force 
for  itself  a  passage  through  the  ground.  In  the  accom- 
panying sketch  (Fig.  58)  the  petiole  of  the  first  leaf 
has  already  partially  straightened  itself,  and  the  blade 
is  beginning  to  unfold.  The  small  second  leaf  ulti- 
mately grows  to  an  equal  size  with  the  first,  but  this 
process  is  effected  at  very  different  rates  in  different 
individuals  :  in  one  instance  the  second  leaf  did  not 
appear  fully  above  the  ground  until  six  weeks  after  the 
first  leaf.  As  the  leaves  in  the  whole  family  of  the 
Acanthaceae  stand  either  opposite  one  another  or  in 
whorls,  and  as  these  are  of  equal  size,  the  great  in- 
equality between  the  first  two  leaves  is  a  singular  fact. 
We  can  see  how  this  inequality  of  development  and 
the  arching  of  the  petiole  could  have  been  gradually 
acquired,  if  they  were  beneficial  to  the  seedlings  by 
favouring  their  emergence  ;  for  with  A.  candelabrum, 
spinosus,  and  latifolius  there  was  great  variability  in  the 
inequality  between  the  two  first  leaves  and  in  the 
arching  of  their  petioles.  In  one  seedling  of  A.  can- 
delabrum the  first  leaf  was  arched  and  nine  times  as 
long  as  the  second,  which  latter  consisted  of  a  mere 
little,  yellowish-white,  straight,  hairy  style.  In  other 
seedlings  the  difference  in  length  between  the  two 
leaves  was  as  3  to  2,  or  as  4  to  3,  or  as  only  '76  to 
•  62  inch.  In  these  latter  cases  the  first  and  taller  leaf 
was  not  properly  arched.  Lastly,  in  another  seedling 
there  was  not  the  least  difference  in  size  between  the 
two  first  leaves,  and  both  of  them  had  their  petioles 
straight ;  their  laminae  were  enfolded  and  pressed 
against  each  other,  forming  a  lance  or  wedge,  by 
which  means  they  had  broken  through  the  ground. 
Therefore  in  different  individuals  of  this  same  species 
of  Acanthus  the  first  pair  of  leaves  breaks  through 
the  ground  by  two  widely  different  methods ;  and  if 


80  HYPOCOTYLS,  EPICOTYLS,  ETC.,         CHAP.  H. 

either  had  proved  decidedly  advantageous  or  disad- 
vantageous, one  of  them  no  doubt  would  soon  have 
prevailed. 

Asa  Gray  has  described  *  the  peculiar  manner  of  ger- 
mination of  three  widely  different  plants,  in  which  the 
hypocotyl  is  hardly  at  all  developed.  These  were  there- 
fore observed  by  us  in  relation  to  our  present  subject. 

Delphinium  nudicaule. — The  elongated  petioles  of 
the  two  cotyledons  are  confluent  (as  are  sometimes 
their  blades  at  the  base),  and  they  break  through  the 
ground  as  an  arch.  They  thus  resemble  in  a  most 
deceptive  manner  a  hypocotyl.  At  first  they  are 
solid,  but  after  a  time  become  tubular  ;  and  the  basal 
•part  beneath  the  ground  is  enlarged  into  a  hollow 
chamber,  within  which  the  young  leaves  are  developed 
without  any  prominent  plumule.  Externally  root- 
hairs  are  formed  on  the  confluent  petioles,  either  a 
little  above,  or  on  a  level  with,  the  plumule.  The 
first  leaf  at  an  early  period  of  its  growth  and  whilst 
within  the  chamber  is  quite  straight,  but  the  petiole 
soon  becomes  arched ;  and  the  swelling  of  this  part 
(and  probably  of  the  blade)  splits  open  one  side  of 
the  chamber,  and  the  leaf  then  emerges.  The  slit 
was  found  in  one  case  to  be  3-2  mm.  in  length,  and 
it  is  seated  on  the  line  of  confluence  of  the  two 
petioles.  The  leaf  when  it  first  escapes  from  the 
chamber  is  buried  beneath  the  ground,  and  now  an 
upper  part  of  the  petiole  near  the  blade  becomes 
arched  in  the  usual  manner.  The  second  leaf  comes 
out  of  the  slit  either  straight  or  somewhat  arched,. but 
afterwards  the  upper  part  of  the  petiole, — certainly  in 
some,  and  we  believe  in  all  cases, — arches  itself  whilst 
forcing  a  passage  through  the  soil. 


*  •  Botanical  Text-Bock,'  1S79,  p.  22. 


CHAP.  II.      BREAKING   THROUGH  THE   GROUND.  81 

Megarrhiza  Calif ornica. — The  cotyledons  of  this 
Gourd  never  free  themselves  from  the  seed-coats  and 
are  hypogean.  Their  petioles  are  completely  con- 
fluent, forming  a  tube  which  terminates  downwards 
in  a  little  solid  point,  consisting  of  a  minute  radicle 
and  hypocotyl,  with  the  likewise  minute  plumule 
enclosed  within  the  base  of  the  tube.  This  structure 
was  well  exhibited  in  an  abnormal  specimen,  in  which 
one  of  the 'two  cotyledons  failed  to  produce  a  petiole, 
whilst  the  other  produced  one  consisting  of  an  open 
semicylinder  ending  in  a  sharp  point,  formed  of  the 
parts  just  described.  As  soon  as  the  confluent 
petioles  protrude  from  the  seed  they  bend  down,  as 
they  are  strongly  geotropic,  and  penetrate  the  ground. 
The  seed  itself  retains  its  original  position,  either 
on  the  surface  or  buried  at  some  depth,  as  the  case 
may  be.  If,  however,  the  point  of  the  confluent 
petioles  meets  with  some  obstacle  in  the  soil,  as 
appears  to  have  occurred  with  the  seedlings  described 
and  figured  by  Asa  Gray,*  the  cotyledons  are  lifted 
up  above  the  ground.  The  petioles  are  clothed  with 
root-hairs  like  those  on  a  true  radicle,  and  they 
likewise  resemble  radicles  in  becoming  brown  when 
immersed  in  a  solution  of  permanganate  of  potassium. 
Our  seeds  were  subjected  to  a  high  temperature,  and 
in  the  course  of  three  or  four  days  the  petioles  pene- 
trated the  soil  perpendicularly  to  a  depth  of  from 
2  to  2J  inches ;  and  not  until  then  did  the  true 
radicle  begin  to  grow.  In  one  specimen  which  was 
closely  observed,  the  petioles  in  7  days  after  their 
first  protrusion  attained  a  length  of  2J  inches,  and  the 
radicle  by  this  time  had  also  become  well  developed. 
The  plumule,  still  enclosed  within  the  tube,  was  now 


*  •  American  Journal  of  Science,'  vol.  xiv.  1877,  p.  21 


82 


HYPOCOTYLS,  EPICOTYLS,  ETC.,        CHAP.  II. 


Fig.  58,  A. 


•3  inch  in  length,  and  was  quite  straight  ;    but  from 
having  increased  in  thickness  it  had  just  begun  to 
split  open  the  lower  part  of  the  petioles  on  one  side, 
along  the  line  of  their  confluence.     By  the  following 
morning  the  upper  part  of  the  plumule  had  arched 
itself  into  a  right   angle,  and  the 
convex  side  or  elbow  had  thus  been 
forced  -out  through  the  slit.     Here 
then  the  arching  of   the    plumule 
plays  the  same  part  as  in  the  case  of 
the  petioles  of  the  Delphinium.     As 
the  plumule  continued  to  grow,  the 
tip   became  more   arched,   and   in 
the  course  of  six  days  it  emerged 
through  the  2J  inches  of  superin- 
cumbent   soil,    still    retaining    its 
arched   form.     After  reaching  the 
surface  it  straightened  itself  in  the 
usual  manner.     In  the  accompany- 
ing figure  (Fig.  58,  A)  we  have  a 
sketch  of   a   seedling   in   this  ad- 
vanced state  of  development;  the 
surface   of  the   ground    being   re- 
Caiifornica  :  presented  by  the  line  Gr  ........  G-. 

jT££  The  germination  of  the  seeds  in 
reduced  to  one-half  their  native  Californian  home  pro- 
**L  Sed-ct'LlT  ceeds  in  a  rather  different  manner, 
the  two  confluent  as  we  infer  from  an  interesting 
r  letter  from  Mr.  Batten,  sent  to  us 
by  Prof.  Asa  Gray.  The  petioles 

,      „  .,  -,  « 

protrude  from  the  seeds  soon  after 
the  autumnal  rains,  and  penetrate  the  ground,  generally 
in  a  vertical  direction,  to  a  depth  of  from  4  to  even 
6  inches.  They  were  found  in  this  state  by  Mr. 
Battan  during  the  Christmas  vacation,  with  the  plu- 


Pi,  plumule  ;  G  ......  0, 

surface  of  soil. 


CHAP.  II.       BREAKING  THROUGH  THE  GROUND.  83 

mules  still  enclosed  within  the  tubes  ;  and  he  remarks 
that  if  the  plumules  had  been  at  once  developed  and 
had  reached  the  surface  (as  occurred  with  our  seeds 
which  were  exposed  to  a  high  temperature),  they 
would  surely  have  been  killed  by  the  frost.  As  it  is 
they  lie  dormant  at  some  depth  beneath  the  surface, 
and  are  thus  protected  from  the  cold ;  and  the  root- 
hairs  on  the  petioles  would  supply  them  with  sufficient 
moisture.  We  shall  hereafter  see  that  many  seedlings 
are  protected  from  frost,  but  by  a  widely  different 
process,  namely,  by  being  drawn  beneath  the  surface 
by  the  contraction  of  their  radicles.  W e  may,  how- 
ever, believe  that  the  extraordinary  manner  of  germi- 
nation of  Megarrhiza  has  another  and  secondary 
advantage.  The  radicle  begins  in  a  few  weeks  to 
enlarge  into  a  little  tuber,  which  then  abounds  with 
starch  and  is  only  slightly  bitter.  It  would  therefore 
be  very  liable  to  be  devoured  by  animals,  were  it  not 
protected  by  being  buried  whilst  young  and  tender,  at  a 
depth  of  some  inches  beneath  the  surface.  Ultimately 
it  grows  to  a  huge  size. 

Ipomoea  leptophylla. — In  most  of  the  species  of  this 
genus  the  hypocotyl  is  well  developed,  and  breaks 
through  the  ground  as  an  arch.  But  the  seeds  of  the 
present  species  in  germinating  behave  like  those  of 
Megarrhiza,  excepting  that  the  elongated  petioles  of 
the  cotyledons  are  not  confluent.  After  they  have 
protruded  from  the  seed,  they  are  united  at  their 
lower  ends  with  the  undeveloped  hypocotyl  and  un- 
developed radicle,  which  together  form  a  point  only 
about  *1  inch  in  length.  They  are  at  first  highly 
geotropic,  and  penetrate  the  ground  to  a  depth  of 
rather  above  half  an  inch.  The  radicle  then  begins 
to  grow.  On  four  occasions  after  .the  petioles  had 
grown  for  a  short  distance  vertically  downwards,  they 


84  HYPOCOTYLS,  EPICOTYLS,  ETC.,        CHAP.  II. 

were  placed  in  a  horizontal  position  in  damp  air  in  the 
dark,  and  in  the  course  of  4  hours  they  again  became 
curved  vertically  downwards,  having  passed  through 
90°  in  this  time.  But  their  sensitiveness  to  geotropism 
lasts  for  only  2  or  3  days;  and  the  terminal  part 
alone,  for  a  length  of  between  *2  and  '4  inch,  is  thus 
sensitive.  Although  the  petioles  of  our  specimens 
did  not  penetrate  the  ground  to  a  greater  depth  than 
about  J  inch,  yet  they  continued  for  some  time  to  grow 
rapidly,  and  finally  attained  the  great  length  of  about 
3  inches.  The  upper  part  is  apogeotropic,  and  there- 
fore grows  vertically  upwards,  excepting  a  short 
portion  close  to  the  blades,  which  at  an  early  period 
bends  downwards  and  becomes  arched,  and  thus 
breaks  through  the  ground.  Afterwards  this  portion 
straightens  itself,  and  the  cotyledons  then  free  them- 
selves from  the  seed-coats.  Thus  we  here  have  in 
different  parts  of  the  same  organ  widely  different  kinds 
of  movement  and  of  sensitiveness ;  for  the  basal  part 
is  geotropic,  the  upper  part  apogeotropic,  and  a  portion 
near  the  blades  temporarily  and  spontaneously  arches 
itself.  The  plumule  is  not  developed  for  some  little 
time  ;  and  as  it  rises  between  the  bases  of  the  parallel 
and  closely  approximate  petioles  of  the  cotyledons, 
which  in  breaking  through  the  ground  have  formed  an 
almost  open  passage,  it  does  not  require  to  be  arched  and 
is  consequently  always  straight.  Whether  the  plumule 
remains  buried  and  dormant  for  a  time  in  its  native 
country,  and  is  thus  protected  from  the  cold  of  winter, 
we  do  not  know.  The  radicle,  like  that  of  the  Megar- 
rhiza,  grows  into  a  tuber-like  mass,  which  ultimately 
attains  a  great  size.  So  it  is  with  Ipomoea  pandurata, 
the  germination  of  which,  as  Asa  Gray  informs  us, 
resembles  that  of  I.  leptopliylla. 

The  following  case  is  interesting  in  connection  with 


CHAP.  II.       BREAKING  THROUGH  THE  GROUND.          85 

the  root-like  nature  of  the  petioles.  The  radicle  of  a 
seedling  was  cut  off,  as  it  was  completely  decayed, 
and  the  two  now  separated  cotyledons  were  planted. 
They  emitted  roots  from  their  bases,  and  continued 
green  and  healthy  for  two  months.  The  blades  of 
both  then  withered,  and  on  removing  the  earth  the 
bases  of  the  petioles  (instead  of  the  radicle)  were 
found  enlarged  into  little  tubers.  Whether  these 
would  have  had  the  power  of  producing  two  in- 
dependent plants  in  the  following  summer,  we  do  not 
know. 

In  Quercus  virens,  according  to  Dr.  Engelmann,* 
both  the  cotyledons  and  their  petioles  are  confluent. 
The  latter  grow  to  a  length  "of  an  inch  or  even 
more;"  and,  if  we  understand  rightly,  penetrate  the 
ground,  so  that  they  must  be  geotropic.  The  nutri- 
ment within  the  cotyledons  is  then  quickly  transferred 
to  the  hypocotyl  or  radicle,  which  thus  becomes 
developed  into  a  fusiform  tuber.  The  fact  of 
tubers  being  formed  by  the  foregoing  three  widely 
distinct  plants,  makes  us  believe  that  their  protection 
from  animals  at  an  early  age  and  whilst  tender,  is  one 
at  least  of  the  advantages  gained  by  the  remark- 
able elongation  of  the  petioles  of  the  cotyledons, 
together  with  their  power  of  penetrating  the  ground 
like  roots  under  the  guidance  of  geotropisrn. 

The  following  cases  may  be  here  given,  as  they  bear 
on  our  present  subject,  though  not  relating  to  seed- 
lings. The  flower-stem  of  the  parasitic  Lathrtea 
squamaria,  which  is  destitute  of  true  leaves,  breaks 
through  the  ground  as  an  arch  ;f  so  does  the  flower- 

*  '  Transact.  St.  Louis  Acad.  ground  cannot  fail  to  be  greatly 

Science,'  vol.  iv.  p.  190.  facilitated  by  the  extraordinary 

t  The  passage  of  the  flower-  quantity  of  water  secreted  at  this 

stem  of  the  Lathrsea  through  the  pt  riod  of  the  year  by  the  subter- 


86 


HYFOCOTYLS,  EPICOTYLS,  ETC.,         CHAP.  II. 


stem  of  the  parasitic  and  leafless  Monotropa  hypopitys. 
With  Hetttiborus  niyer,  the  ilower-stems,  which  rise  up 
independently  of  the  leaves,  likewise  break  through 
the  ground  as  arches.  This  is  also  the  case  with  the 
greatly  elongated  flower-stems,  as  well  as  with  the 
petioles  of  Epimedlum  pinnatum.  So  it  is  with  the 
petioles  of  Ranunculus  fiearia,  when  they  have  to  break 
through  the  ground,  but  when  they  arise  from  the 
summit  of  the  bulb  above  ground,  they  are  from  the 
first  quite  straight ;  and  this  is  a  fact  which  deserves 
notice.  The  rachis  of  the  bracken  fern  (Pteris  aqui- 
Una),  and  of  some,  probably  many,  other  ferns,  like- 
wise rises  above  ground  under  the  form  of  an  arch. 
No  doubt  other  analogous  instances  could  be  found  by 
careful  search.  In  all  ordinary  cases  of  bulbs,  rhizomes, 


ranean  scale-like  leaves  ;  not  that 
there  is  any  reason  to  suppose 
that  the  secretion  is  a  special 
adaptation  for  this  purpose:  it 
probably  follows  from  the  great 
quantity  of  sap  absorbed  in  the 
early  spring  by  the  parasitic  roots. 
After  a  long  period  without  any 
rain,  the  earth  had  become  light- 
coloured  and  wry  dry,  but  it  was 
dark  coloured  and  damp,  even  in 
parts  quite  wet.  for  a  distance  of 
al  least  six  inches  all  round  each 
flower-stem.  The  water  is  secreted 
by  glands  (described  by  Colin, 
'Bericlit.  Bot.  Sect,  der  Schle- 
sischen  Gesell.,'  1876,  p.  113) 
which  line  the  longitudinal 
channels  running  through  each 
scale-like  leaf.  A  large  plant  was 
dug  up,  washed  so  as  to  remove 
the  earth,  left  for  some  time  to 
drain,  and  then  placed  in  the 
evening  on  a  dry  glass-plate, 
covered  with  a  bell-glass,  and  by 
next  morning  it  lal  secreted  a 
large  pool  of  water.  The  pLte 
was  wiped  dry,  and  in  the  course 
of  the  succeeding  7  or  8  hours 


another  little  pool  was  secreted, 
and  after  16  additional  hours 
several  large  drops.  A  smaller 
plant  was  washed  and  placed  in  a 
large  jar,  which  was  left  inclined 
for  au  hour,  by  which  time  no 
more  water  drained  off.  The  jar 
was  then  placed  upright  and 
closed :  after  23  hours  twodmchms 
of  water  were  collected  from  the 
bottom,  and  a  little  more  after  2o 
additional  hours.  The  flower- 
stems  were  now  cut  off,  fur  they 
do  not  secrete,  and  the  subter- 
ranean part  of  the  plant  was  found 
to  weigh  106-8  grams  (1611 
grains),  and  the  water  seer*  ted 
during  the  48 .  hours  weighed 
1T9  grams  (183  grains), — that  is, 
one-ninth  of  the  whole  weight  of 
the  plant,  excluding  the  flmver- 
stems.  \Ve  should  rememlx  r  that 
plants  in  a  state  of  nature  would 
probably  secrete  in  48  hours  much 
more  than  the  above  large  amount, 
for  their  roots  would  continue  all 
the  time  absorbing  sap  from  the 
plant  on  which  they  were  para- 
sitic. 


CHAP.  II.       BREAKING  THROUGH  THE   GROUND.  87 

root-stocks,  &c.,  buried  beneath  the  ground,  the  surface 
is  broken  by  a  cone  formed  by  the  young  imbricated- 
leaves,  the  combined  growth  of  which  gives  them  force 
sufficient  for  the  purpose. 

With  germinating  monocotyledonous  seeds,  of 
which,  however,  we  did  not  observe  a  large  number, 
the  plumules,  for  instance,  those  of  Asparagus  and 
Canna,  are  straight  whilst  breaking  through  the  ground. 
With  the  Gramineae,  the  sheath-like  •  cotyledons  are 
likewise  straight ;  they,  however,  terminate  in  a  sharp 
crest,  which  is  white  and  somewhat  indurated ;  and  this 
structure  obviously  facilitates  their  emergence  from 
the  soil :  the  first  true  leaves  escape  from  the  sheath 
through  a  slit  beneath  the  chisel-like  apex  and  at 
right  angles  to  it.  In  the  case  of  the  onion  (Allium 
cepa)  we  again  meet  with  an  arch  ;  the  leaf-like  coty- 
ledon being  abruptly  bowed,  when  it  breaks  through 
the  ground,  with  the  apex  still  enclosed  within  the 
seed-coats.  The  crown  of  the  arch,  as  previously 
described,  is  developed  into  a  white  conical  pro- 
tuberance, which  we  may  safely  believe  to  be  a 
special  adaptation  for  this  office. 

The  fact  of  so  many  organs  of  different  kinds — 
hypocotyls  and  epicotyls,  the  petioles  of  some  coty- 
ledons and  of  some  first  leaves,  the  cotyledons  of 
the  onion,  the  rachis  of  some  ferns,  and  some  flower- 
stems — being  all  arched  whilst  they  break  through 
the  ground,  shows  how  just  are  Dr.  Haberlandt's  * 
remarks  on  the  importance  of  the  arch  to  seedling 
plants.  He  attributes  its  chief  importance  to  the 
upper,  young,  and  more  tender  parts  of  the  hypocotyl 


*  •  Die  Schutzeinrichtunj;en  in  though  our  observations  lead  us 

der     Eutwickelmig    der     Keim-  to  differ  on  some  points  from  tho 

pflanze,'  1877.    We  have  learned  author, 
much  from  this  interesting  essay, 

5 


88  HYPOCOTYLS,  EPICOTYLS,  ETC.,         CHAP   II. 

or  epicotyl,  being  thus  saved  from  abrasion  and 
•pressure  whilst  breaking  through  the  ground.  But 
we  think  that  some  importance  may  be  attributed  to 
the  increased  force  gained  by  the  hypocotyl,  epicotyl, 
or  other  organ  by  being  at  first  arched  ;  for  both  legs  of 
the  arch  increase  in  length,  and  both  have  points  of 
resistance  as  long  as  the  tip  remains  enclosed  within 
the  seed-coats;  and  thus  the  crown  of  the  arch  is 
pushed  up  through  the  earth  with  twice  as  much  force 
as  that  which  a  straight  hypocotyl,  &c.,  could  exert. 
As  soon,  however,  as  the  upper  end  has  freed  itself, 
all  the  work  has  to  be  done  by  the  basal  leg.  In 
the  case  of  the  epicotyl  of  the  common  bean,  the 
basal  leg  (the  apex  having  freed  itself  from  the  seed- 
coats)  grew  upwards  with  a  force  sufficient  to  lift  a 
thin  plate  of  zinc,  loaded  with  12  ounces.  Two  more 
ounces  were  added,  and  the  14  ounces  were  lifted  up 
to  a  very  little  height,  and  then  the  epicotyl  yielded 
and  bent  to  one  side. 

With  respect  to  the  primary  cause  of  the  arching 
process,  we  long  thought  in  the  case  of  many  seedlings 
that  this  might  be  attributed  to  the  manner  in  which 
the  hypocotyl  or  epicotyl  was  packed  and  curved 
within  the  seed-coats  ;  and  that  the  arched  shape  thus 
acquired  was  merely  retained  until  the  parts  in  question 
reached  the  surface  of  the  ground.  But  it  is  doubtful 
whether  this  is  the  whole  of  the  truth  in  any  case. 
For  instance,  with  the  common  bean,  the  epicotyl  or 
plumule  is  bowed  into  an  arch  whilst  breaking  through 
the  seed-coats,  as  shown  in  Fig.  59  (p.  92).  The 
plumule  first  protrudes  as  a  solid  knob  (e  in  A),  which 
after  twenty-four  hours'  growth  is  seen  (e  in  B)  to  be 
the  crown  of  an  arch.  Nevertheless,  with  several 
beans  which  germinated  in  damp  air,  and  had  other- 
wise been  treated  in  an  unnatural  manner,  little 


CHAP.  II.        BKEAKING  THKOUGH  THE   GEOUND.          89 

plumules  were  developed  in  the  axils  of  the  petioles 
of  both  cotyledons,  and  these  were  as  perfectly  arched 
as  the  normal  plumule;  yet  they  had  not  been  sub- 
jected to  any  confinement  or  pressure,  for  the  seed- 
coats  were  completely  ruptured,  and  they  grew  in  the 
open  air.  This  proves  that  the  plumule  has  an  innate 
or  spontaneous  tendency  to  arch  itself. 

In  some  other  cases  the  hypocotyl  or  epicotyl  pro- 
trudes from  the  seed  at  first  only  slightly  bowed ;  but 
the  bowing  afterwards  increases  independently  of  any 
constraint.  The  arch  is  thus  made  narrow,  with  the 
two  legs,  which  are  sometimes  much  elongated,  parallel 
and  close  together,  and  thus  it  becomes  well  fitted 
for  breaking  through  the  ground. 

With  many  kinds  of  plants,  the  radicle,  whilst  still 
enclosed  within  the  seed  and  likewise  after  its  first  pro- 
trusion, lies  in  a  straight  line  with  the  future  hypocotyl 
and  with  the  longitudinal  axis  of  the  cotyledons.  This 
is  the  case  with  CucurUta  ovifera ;  nevertheless,  in 
whatever  position  the  seeds  were  buried,  the  hypocotyl 
always  came  up  arched  in  one  particular  direction. 
Seeds  were  planted  in  friable  peat  at  a  depth  of  about 
an  inch  in  a  vertical  position,  with  the  end  from  which 
the  radicle  protrudes  downwards.  Therefore  all  the 
parts  occupied  the  same  relative  positions  which 
they  would  ultimately  hold  after  the  seedlings  had 
risen  clear  above  the  surface.  Notwithstanding  this 
fact,  the  hypocotyl  arched  itself;  and  as  the  arch 
grew  upwards  through  the  peat,  the  buried  seeds  were 
turned  either  upside  down,  or  were  laid  horizontally, 
being  afterwards  dragged  above  the  ground.  Ulti- 
mately the  hypocotyl  straightened  itself  in  the  usual 
manner;  and  now  after  all  these  movements  the 
several  parts  occupied  the  same  position  relatively  to 
one  another  and  .to  the  centre  of  the  earth,  which  they 


90  HYPOCOTYLS,  EPICOTYLS,  ETC.,          CHAP.  II. 

had  done  when  the  seeds  were  first  buried.  But  it  may 
be  argued  in  this  and  other  such  cases  that,  as  the 
hypocotyl  grows  up  through  the  soil,  the  seed  will 
almost  certainly  be  tilted  to  one  side ;  and  then 
from  the  resistance  wrhich  it  must  offer  during  its 
further  elevation,  the  upper  part  of  the  hypocotyl  will 
be  doubled  down  and  thus  become  arched.  This  view 
seems  the  more  probable,  because  with  Ranunculus 
ficaria  only  the  petioles  of  the  leaves  which  forced 
a  passage  through  the  earth  were  arched ;  and  not 
those  which  arose  from  the  summits  of  the  bulbs  above 
the  ground.  Nevertheless,  this  explanation  does  not 
apply  to  the  Cucurbita,  for  when  germinating  seeds 
were  suspended  in  damp  air  in  various  positions  by 
pins  passing  through  the  cotyledons,  fixed  to  the 
inside  of  the  lids  of  jars,  in  which  case  the  hypo- 
cotyls  were  not  subjected  to  any  friction  or  constraint, 
yet  the  upper  part  became  spontaneously  arched. 
This  fact,  moreover,  proves  that  it  is  not  the  weight 
of  the  cotyledons  which  causes  the  arching.  Seeds 
of  Helianthus  annum  and  of  two  species  of  Ipomoaa 
(those  of  I.  lona  nox  being  for  the  genus  large 
and  heavy)  were  pinned  in  the  same  manner, 
and  the  hypocotyls  became  spontaneously  arched ; 
the  radicles,  which  had  been  vertically  dependent, 
assumed  in  consequence  a  horizontal  position.  In 
the  case  of  Ipomcea  leptophylla  it  is  the  petioles  of  the 
cotyledons  which  become  arched  whilst  rising  through 
the  ground ;  and  this  occurred  spontaneously  when 
the  seeds  were  fixed  to  the  lids  of  jars. 

It  may,  however,  be  suggested  with  some  degree  of 
probability  that  the  arching  was  aboriginally  caused 
by  mechanical  compulsion,  owing  to  the  confinement 
of  the  parts  in  question  within  the  seed-coats,  or  to 
friction  whilst  they  were  being  dragged  upwards.  But 


CHAP.  II.       BREAKING   THROUGH  THE   GROUND.  91 

if  this  is  so,  we  must  admit  from  the  cases  just  given, 
that  a  tendency  in  the  upper  part  of  the  several 
specified  organs  to  bend  downwards  and  thus  to  be- 
come arched,  has  now  become  with  many  plants  firmly 
inherited.  The  arching,  to  whatever  cause  it  may  be 
due,  is  the  result  of  modified  circumnutation,  through 
increased  growth  along  the  convex  side  of  the  part ; 
such  growth  being  only  temporary,  for  the  part  always 
straightens  itself  subsequently  by  increased  growth 
along  the  concave  side,  as  will  hereafter  be  described. 

It  is  a  curious  fact  that  the  hypocotyls  ~of  some 
plants,  which  are  but  little  developed  and  which 
never  raise  their  cotyledons  above  the  ground,  never- 
theless inherit  a  slight  tendency  to  arch  themselves, 
although  this  movement  is  not  of  the  least  use  to 
them.  We  refer  to  a  movement  observed  by  Sachs 
in  the  hypocotyls  of  the  bean  and  some  other  Legumi- 
nosse,  and  which  is  shown  in  the  accompanying  figure 
(Fig.  59),  copied  from  his  Essay.*  The  hypocotyl 
and  radicle  at  first  grow  perpendicularly  downwards, 
as  at  A,  and  then  bend,  often  in  the  course  of  24  hours, 
into  the  position  shown  at  B.  As  we  shall  here- 
after often  have  to  recur  to  this  movement,  we  will,  for 
brevity  sake,  call  it  "  Sachs'  curvature."  At  first  sight 
it  might  be  thought  that  the  altered  position  of  the 
radicle  in  B  was  wholly  due  to  the  outgrowth  of  the 
epicotyl  (e),  the  petiole  (p)  serving  as  a  hinge ;  and 
it  is  probable  that  this  is  partly  the  cause ;  but  the 
hypocotyl  and  upper  part  of  the  radicle  themselves 
become  slightly  curved. 

The  above  movement  in  the  bean  was  repeatedly 
seen  by  us  ;  but  our  observations  were  made  chiefly  on 
Phaseolus  multiflorus,  the  cotyledons  of  which  are  like- 


'  Arbeiten  des  bot.  Instit.  Wiirzburg,'  vol.  i.  1873,  p.  403. 


92 


HYPOCOTYLS,   EPICOTYLS,  ETC.,          CHAP.  II. 


wise  hypogean.  Some  seedlings  with  well-developed 
radicles  were  first  immersed  in  a  solution  of  perman- 
ganate of  potassium ;  and,  judging  from  the  changes 
of  colour  (though  these  were  not  very  clearly  defined), 
the  hypocotyl  is  about  *3  inch  in  length.  Straight, 
thin,  black  lines  of  this  length  were  now  drawn  from 
the  bases  of  the  short  petioles  along  the  hypocotyls 


F:g.  59. 


u 


Vicia  faba :  germinating  seeds,  suspended  in  damp  air :  A,  with  radicle 
growing  perpendicularly  downwards  ;  B,  the  same  bean  after  24  hours 
and  after  the  radicle  has  curved  itself;  r,  radicle;  7t,  short  hypocotyl  ; 
e,  epicotyi  appearing  as  a  knob  in  A  and  as  an  arch  in  B ;  p,  petiole  of 
the  cotyledon,  the  latter  enclosed  within  the  seed-coats. 

of  23  germinating  seeds,  which  were  pinned  to  the 
lids  of  jars,  generally  with  the  hilum  downwards,  and 
with  their  radicles  pointing  to  the  centre  of  the 
earth.  After  an  interval  of  from  24  to  48  hours  the 
black  lines  on  the  hypocotyls  of  16  out  of  the  23 
seedlings  became  distinctly  curved,  but  in  very 
various  degrees  (namely,  with  radii  between  20  and 


CHAP.  II.       BREAKING   THROUGH   THE   GROUND.  93 

80  mm.  on  Sachs'  cyclometer)  in  the  same  relative 
direction  as  shown  at  B  in  Fig.  59.  As  geotropism 
will  obviously  tend  to  check  this  curvature,  seven 
seeds  were  allowed  to  germinate  with  proper  pre- 
cautions for  their  growth  in  a  klinostat,*  by  which 
means  geotropism  was  eliminated.  The  position  of  the 
hypocotyls  was  observed  during  four  successive  days, 
and  they  continued  to  bend  towards  the  hilum  and 
lower  surface  of  the  seed.  On  the  fourth  day  they 
were  deflected  by  an  average  angle  of  63°  from  a  line 
perpendicular  to  the  lower  surface,  and  were  therefore 
considerably  more  curved  than  the  hypocotyl  and 
radicle  in  the  bean  at  B  (Fig.  59),  though  in  the  same 
relative  direction. 

It  will,  we  presume,  be  admitted  that  all  leguminous 
plants  with  hypogean  cotyledons  are  descended  from 
forms  which  once  raised  their  cotyledons  above  the 
ground  in  the  ordinary  manner ;  and  in  doing  so,  it  is 
certain  that  their  hypocotyls  would  have  been  abruptly 
arched,  as  in  the  case  of  every  other  dicotyledonous 
plant.  This  is  especially  clear  in  the  case  of  Phaseolus, 
for  out  of  five  species,  the  seedlings  of  which  we 
observed,  namely,  P.  multiflorus,  caracalla,  vulgaris, 
Hernandesii  and  Eoxburgliii  (inhabitants  of  the  Old 
and  New  Worlds),  the  three  last-named  species  have 
well-developed  hypocotyls  which  break  through  the 
ground  as  arches.  Now,  if  we  imagine  a  seedling  of 
the  common  bean  or  of  P.  multiflorus,  to  behave  as  its 
progenitors  once  did,  the  hypocotyl  (A,  Fig.  59),  in 
whatever  position  the  seed  may  have  been  buried, 
would  become  so  much  arched  that  the  upper  part 
would  be  doubled  down  parallel  to  the  lower  part ;  and 


*  An  instrument  devised  by  on  which  the  plant  under  observa- 
Sachs,  consisting  essentially  of  a  tion  is  supported  :  see  '"Wiirzburg 
slowly  revolving  horizontal  axis.  Arbeiten,'  1879,  p.  209. 


RUDIMENTARY   COTYLrEDOXS. 


CHAP.  II. 


this  is  exactly  the  kind  of  curvature  which  actually 
occurs  in  these  two  plants,  though  to  a  much  less 
degree.  Therefore  we  can  hardly  doubt  that  their 
short  hypocotyls  have  retained  by  inheritance  a  ten- 
dency to  curve  themselves  in  the  same  manner  as  they 
did  at  a  former  period,  when  this  movement  was  highly 
important  to  them  for  breaking  through  the  ground, 
though  now  rendered  useless  by  the  cotyledons  being 
hypogean.  Rudimentary  structures  are  in  most  cases 
highly  variable,  and  we  might  expect  that  rudimentary 
or  obsolete  actions  would  be  equally  so ;  and  Sachs' 
curvature  varies  extremely  in  amount,  and  sometimes 
altogether  fails.  This  is  the  sole  instance  known  to 
us  of  the  inheritance,  though  in  a  feeble  degree,  of 
movements  which  have  become  superfluous  from 
changes  which  the  species  has  undergone. 


Fig.  60. 


Rudimentary   Cotyledons. — A  few   remarks   on  this 
subject  may  be  here  interpolated.     It  is  well  known 

that  some  dicotyle- 
donous plants  produce 
only  a  single  cotyle- 
don; for  instance,  cer- 
tain species  of  Ranun- 
culus, Corydalis,  Chae- 
rophyllum ;  and  we 
will  here  endeavour  to 
show  that  the  loss  of 
one  or  both  cotyle- 
dons is  apparently  due 

Citrus  aurantium:    two  young  seedlings:  ^o     a   store     of     nutri- 

c.  larger  cotyledon :  c',  smaller  cotyle-  ,  ,     .         1-1 

don  ;  h,  thickened  hypocotyl ;  r,  radicle,  ment  being  laid  Up  m 

In  A  the  epicotyl  is  still  arched,  in  B  it  some  other  part,  as  in 

has  become  erect.  ,        ,                f  •, 

the  hypocotyl  or  one 
of  the  two  cotyledons,  or  one  of  the  secondary  radicles. 


CHAP.  II. 


KUDIMENTAKY   COTYLEDONS. 


95 


Fig.  61. 


With  the  orange  (Citrus  aurantium)  the  cotyledons  are 
hypogean,  and  one  is  larger  than  the  other,  as  may 
be  seen  in  A  (Fig.  60).  In  B  the  inequality  is  rather 
greater,  and  the  stem  has  grown  between  the  points 
of  insertion  of  the  two  petioles,  so  that  they  do  not 
stand  opposite  to  one  another;  in  another  case  the 
separation  amounted  to  one-fifth  of  an  inch.  The 
smaller  cotyledon  of  one  seedling 
was  extremely  thin,  and  not  half 
the  length  of  the  larger  one,  so  that 
it  was  clearly  becoming  rudimen- 
tary.* In  all  these  seedlings  the 
hypocotyl  was  enlarged  or  swollen. 
With  Abronia  umbettata  one  of 
the  cotyledons  is  quite  rudimen- 
tary, as  may  be  seen  (c)  in  Fig.  61. 
In  this  specimen  it  consisted  of  a 
little  green  flap,  TVth  inch  in 
length,  destitute  of  a  petiole  and 
covered  with  glands  like  those  on 
the  fully  developed  cotyledon  (c). 

At    first    it    Stood    Opposite    to    the    Abronia  umbellate  : 

larger  cotyledon  ;  but  as  the  petiole      ling  twice  natural  size: 

° ,        ,    f ,          .    '  ,     .       \         Al          c,  cotyledon ;  c',  rudi- 

of  the  latter   increased   in   length      mentaiy  cotyledon ;  h, 

and  grew  in    the   same   line  with 

the    hypocotyl   (7i),    the   rudiment 

appeared  in  older   seedlings  as   if 

seated  some  way  down  the  hypocotyl.     With  Abronia 

arenaria  there  is  a  similar  rudiment,  which  in  one 


enlarged  hypocotyl, 
with  a  heel  or  projec- 
tion (/«')  at  the  lower 
end ;  r,  radicle. 


*  In  Pachira  aquatica,  ns  de- 
scribed by  Mr.  K.  I.  Lynch 
('Journal  Linn.  Soc.  Bot.'  vol. 
xvii.  1878,  p.  147),  one  of  the 
hypogean  cotyledons  is  of  im- 
mense size ;  the  other  is  small 
and  soon  falls  off;  the  pair  do  not 
always  stand  opposite.  In  another 


and  very  different  water-plant, 
Trapa  wit  cms,  one  of  the  cotyle- 
dons, filled  with  farinaceous 
matter,  is  much  larger  than  the 
other,  which  is  scarcely  visible, 
as  is  stated  by  Aug.  de  Candolle, 
'  Physiologic  Ve'g.'  torn.  ii.  p.  834, 
1832. 


96         RUDIMENTARY  COTYLEDONS.    CUAP.  II. 


specimen  was  only  -r^-th  and  in  another  ^th  inch  in 
length;  it  ultimately  appeared  as  if  seated  halfway 
down  the  hypocotyl.  In  both  these  species  the  hypo- 
cotyl  is  so  much  enlarged,  especially  at  a  very  early 
age,  that  it  might  almost  be  called  a  corm.  The  lower 
end  forms  a  heel  or  projection,  the  use  of  which  will 
hereafter  be  described. 

In  Cyclamen  Persicum  the  hypocotyl,  even  whilst  still 
within  the  seed,  is  enlarged  into  a  regular  corm,*  and 
only  a  single  cotyledon  is  at  first  developed  (see  former 
Fig.  57.)  With  Ranunculus  ficaria  two  cotyledons  are 
never  produced,  and  here  one  of  the  secondary  radicles 
is  developed  at  an  early  age  into  a  so-called  bulb.f 
Again,  certain  species  of  Chserophylluni  and  Corydalis 
produce  only  a  single  cotyledon  ;{  in  the  former  the 
hypocotyl,  and  in  the  latter  the  radicle  is  enlarged, 
according  to  Irmisch,  into  a  bulb. 

In  the  several  foregoing  cases  one  of  the  cotyledons 
is  delayed  in  its  development,  or  reduced  in  size,  or 
rendered  rudimentary,  or  quite  aborted  ;  but  in  other 
cases  both  cotyledons  are  represented  by  mere  rudi- 
ments. With  Opuntia  lasilaris  this  is  not  the  case, 
for  both  cotyledons  are  thick  and  large,  and  the 
hypocotyl  shows  at  first  no  signs  of  enlargement  ;  but 
afterwards,  when  the  cotyledons  have  withered  and  dis- 
articulated themselves,  it  becomes  thickened,  and  from 
its  tapering  form,  together  with  its  smooth,  tough, 
brown  skin,  appears,  when  ultimately  drawn  down  to 
some  depth  into  the  soil,  like  a  root.  On  the  other 

*  Dr.  H.  Gressner,  '  Bot.  Zei-  Vauclier's  account  ('  Hist.   Phys. 

tung,'  1874,  p.  824.  des  Plantes  d'Europe,'  torn.  i.  1841, 

t  Irmisch,  *  Beitrage  zur  Mor-  p.  149)  of  the  germination  of  the 

phologie  der  Pflanzen,'  1854,  pp.  seeds  of  several  species  of  Corv- 

11,  12;  'Bot.  Zeitung,'  1874,  p.  dalis,  that  the  bulb  or  tubercule 

805.  begins  to  be  formed   at  an  ex- 

J  Delpino,  *  Rivista  Botanica,'  tremely  early  age. 
1877,  p.  21.    It  is  evident  from 


CHAP.  II.  EUDIMENTARY   COTYLEDONS.  97 

hand,  with  several  other  Cactese,  the  hypocotyl  is  from 
the  first  much  enlarged,  and  both  cotyledons  are 
almost  or  quite  rudimentary.  Thus  with  Cereus  Land- 
beckii  two  little  triangular  projections,  representing  the 
cotyledons,  are  narrower  than  the  hypocotyl,  which  is 
pear-shaped,  with  the  point  downwards.  In  Rliipsalis 
cassytha  the  cotyledons  are  represented  by  mere  points 
on  the  enlarged  hypocotyl.  In  Echinocactus  viridescens 
the  hypocotyl  is  globular,  with  two  little  prominences 
on  its  summit.  In  Pilocereus  Houlletii  the  hypocotyl, 
much  swollen  in  the  upper  part,  is  merely  notched  on 
the  summit ;  and  each  side  of  the  notch  evidently  repre- 
sents a  cotyledon.  Stapelia  sarpedon,  a  member  of  the 
very  distinct  family  of  the  Asclepiadeae,  is  fleshy  like 
a  cactus  ;  and  here  again  the  upper  part  of  the  flattened 
hypocotyl  is  much  thickened  and  bears  two  minute  coty- 
ledons, which,  measured  internally,  were  only  *15  inch 
in  length,  and  in  breadth  not  equal  to  one-fourth  of  the 
diameter  of  the  hypocotyl  in  its  narrow  axis ;  yet  these 
minute  cotyledons  are  probably  not  quite  useless,  for 
when  the  hypocotyl  breaks  through  the  ground  in  the 
form  of  an  arch,  they  are  closed  or  pressed  against  one 
another,  and  thus  protect  the  plumule.  They  after- 
wards open. 

From  the  several  cases  now  given,  which  refer  to 
widely  distinct  plants,  we  may  infer  that  there  is  some 
close  connection  between  the  reduced  size  of  one  or 
both  cotyledons  and  the  formation,  by  the  enlargement 
of  the  hypocotyl  or  of  the  radicle,  of  a  so-called  bulb. 
But  it  may  be  asked,  did  the  cotyledons  first  tend  to 
abort,  or  did  a  bulb  first  begin  to  be  formed?  As 
all  dicotyledons  naturally  produce  two  well-developed 
cotyledons,  whilst  the  thickness  of  the  hypocotyl  and 
of  the  radicle  differs  much  in  different  plants,  it  seems 
probable  that  these  latter  organs  first  became  from 


98  CIRCUMNUTATIXG  MOVEMENTS  OF      CHAP.  II. 

some  cause  thickened — in  several  instances  apparently 
in  correlation  with  the  fleshy  nature  of  the  mature 
plant — so  as  to  contain  a  store  of  nutriment  sufficient 
for  the  seedling,  and  then  that  one  or  both  cotyledons, 
from  being  superfluous,  decreased  in  size.  It  is  not 
surprising  that  .one  cotyledon  alone  should  sometimes 
have  been  thus  affected,  for  with  certain  plants,  for 
instance  the  cabbage,  the  cotyledons  are  at  first  of 
unequal  size,  owing  apparently  to  the  manner  in  which 
they  are  packed  within  the  seQd.  It  does  not,  how- 
ever, follow  from  the  above  connection,  that  whenever 
a  bulb  is  formed  at  an  early  age,  one  or  both  coty- 
ledons will  necessarily  become  superfluous,  and  conse- 
quently more  or  less  rudimentary.  Finally,  these 
cases  offer  a  good  illustration  of  the  principle  of  com- 
pensation or  balancement  of  growth,  or,  as  Goethe 
expresses  it,  "  in  order  to  spend  on  one  side,  Nature 
is  forced  to  economise  on  the  other  side." 

Circumnutation  and  other  movements  of  Hypocotyls' 
and  Epicotyls,  whilst  still  arched  and  buried  beneath 
the  ground,  and  whilst  breaking  through  it. — According 
to  the  position  in  which  a  seed  may  chance  to 
have  been  buried,  the  arched  hypocotyl  or  epicotyl 
will  begin  to  protrude  in  a  horizontal,  a  more  or 
less  inclined,  or  in  a  vertical  plane.  Except  when 
already  standing  vertically  upwards,  both  legs  of  the 
arch  are  acted  on  from  the  earliest  period  by  apo- 
geotropism.  Consequently  they  both  bend  upwards, 
until  the  arch  becomes  vertical.  During  the  whole  of 
this  process,  even  before  the  arch  has  broken  through 
the  ground,  it  is  continually  trying  to  circumnutate 
to  a  slight  extent ;  as  it  likewise  does  if  it  happens  at 
first  to  stand  vertically  up, — all  which  cases  have 
been  observed  and  described,  more  or  less  fully,  in 
the  last  chapter.  After  the  arch  has  grown  to  some 


CHAP.  II.       HYPOCOTYLS,   ETC.,  WHILST   ARCHED.        99 

height  upwards,  the  basal  part  ceases  to  circumimtate, 
whilst  the  upper  part  continues  to  do  so. 

That  an  arched  hypocotyl  or  epicotyl,  with  the  two 
legs  fixed  in  the  ground,  should  be  able  to  cir- 
cumnutate,  seemed  to  us,  until  we  had  read  Prof. 
Wiesner's  observations,  an  inexplicable  fact.  He  has 
shown*  in  the  case  of  certain  seedlings,  whose  tips 
are  bent  downwards  (or  which  nutate),  that  whilst  the 
posterior  side  of  the  upper  or  dependent  portion  grows 
quickest,  the  anterior  and  opposite  side  of  the  basal 
portion  of  the  same  internode  grows  quickest ;  these 
two  portions  being  separated  by  an  indifferent  zone, 
where  the  growth  is  equal  on  all  sides.  There  may 
even  be  more  than  one  indifferent  zone  in  the  same 
internode ;  and  the  opposite  sides  of  the  parts  above 
and  below  each  such  zone  grow  quickest.  This  pecu- 
liar manner  of  growth  is  called  by  Wiesner  "un- 
dulatory  nutation."  Circumnutation  depends  on  one 
side  of  an  organ  growing  quickest  (probably  preceded 
by  increased  turgescence),  and  then  another  side, 
generally  almost  the  opposite  one,  growing  quickest. 
Now  if  we  look  at  an  arch  like  this  f|  and  suppose 
the  whole  of  one  side— we  will  say  the  whole  convex 
side  of  both  legs — to  increase  in  length,  this  would 
not  cause  the  arch  to  bend  to  either  side.  But  if  the 
outer  side  or  surface  of  the  left  leg  were  to  increase 
in  length  the  arch  would  be  pushed  over  to  the  right, 
and  this  would  be  aided  by  the  inner  side  of  the 
right  leg  increasing  in  length.  If  afterwards  the 
process  were  reversed,  the  arch  would  be  pushed  over 
to  the  opposite  or  left  side,  and  so  on  alternately, — 
that  is,  it  would  circumnutate.  As  an  arched  hypo- 


*  'Die    undulirende   Nutation      Also    published    separately,  seo 
der  Internodien,'    Akad.  der  Wis-       p.  32. 
sench.  (Vienna),  Jan.  17tb,  1878. 


100          CIKCUMNUTATING  MOVEMENTS  OF       CHAP.  II. 

cotyl,  with  the  two  legs  fixed  in  the  ground,  certainly 
circumnutates,  and  as  it  consists  of  a  single  internode, 
we  may  conclude  that  it  grows  in  the  manner  de- 
scribed by  Wiesner.  It  may  be  added,  that  the  crown 
of  the  arch  does  not  grow,  or  grows  very  slowly,  for 
it  does  not  increase  much  in  breadth,  whilst  the  arch 
itself  increases  greatly  in  height. 

The  circumnutating  movements  of  arched  hypo- 
cotyls  and  epicotyls  can  hardly  fail  to  aid  them  in 
breaking  through  the  ground,  if  this  be  damp  and 
soft;  though  no  doubt  their  emergence  depends 
mainly  on  the  force  exerted  by  their  longitudinal 
growth.  Although  the  arch  circumnutates  only  to  a 
slight  extent  and  probably  with  little  force,  yet  it  is 
able  to  move  the  soil  near  the  surface,  though  it  may 
not  be  able  to  do  so  at  a  moderate  depth.  A  pot  with 
seeds  of  Solanum  paUnacanthum,  the  tall  arched  hypo- 
cptyls  of  which  had  emerged  and  were  growing  rather 
slowly,  was  covered  with  fine  argillaceous  sand  kept 
damp,  and  this  at  first  closely  surrounded  the  bases  of 
the  arches ;  but  soon  a  narrow  open  crack  was  formed 
round  each  of  them,  which  could  be  accounted  for 
only  by  their  having  pushed  away  the  sand  on  all 
sides ;  for  no  such  cracks  surrounded  some  little  sticks 
and  pins  which  had  been  driven  into  the  sand.  It 
has  already  been  stated  that  the  cotyledons  of  Phalaris 
and  Avena,  the  plumules  of  Asparagus  and  the  hypo- 
cotyls  of  Brassica,  were  likewise  able  to  displace  the 
same  kind  of  sand,  either  whilst  simply  circumnu- 
tating or  whilst  bending  towards  a  lateral  light. 

As  long  as  an  arched  hypocotyl  or  epicotyl  remains 
buried  beneath  the  ground,  the  two  legs  cannot  sepa- 
rate from  one  another,  except  to  a  slight  extent  from 
the  yielding  of  the  soil;  but  as  soon  as  the  arch 
rises  above  the  ground,  or  at  an  earlier  period  if 


CHAP.  II.       HYPOCOTYLS,   ETC.,  WHILST  ARCHED.      101 

the  pressure  of  the  surrounding  earth  be  artificially 
removed,  the  arch  immediately  begins  to  straighten 
itself.  This  no  doubt  is  due  to  growth  along  the 
whole  inner  surface  of  both  legs  of  the  arch;  such 
growth  being  checked  or  prevented,  as  long  as  the  two 
legs  of  the  arch  are  firmly  pressed  together.  When  the 
earth  is  removed  all  round  an  arch  and  the  two  legs 
are  tied  together  at  their  bases,  the  growth  on  the 
under  side  of  the  crown  causes  it  after  a  time  to 
become  much  natter  and  broader  than  naturally 
occurs.  The  straightening  process  consists  of  a  mo- 
dified form  of  circumnutation,  for  the  lines  described 
during  this  process  (as  with  the  hypocotyl  of  Brassica, 
and  the  epicotyls  of  Yicia  and  Corylus)  were  often 
plainly  zigzag  and  sometimes  looped.  After  hypo- 
cotyls  or  epicotyls  have  emerged  from  the  ground, 
they  quickly  become  perfectly  straight.  No  trace  is 
left  of  their  former  abrupt  curvature,  excepting  in  the 
case  of  Allium  cepa,  in  which  the  cotyledon  rarely 
becomes  quite  straight,  owing  to  the  protuberance 
developed  on  the  crown  of  the  arch. 

The  increased  growth  along  the  inner  surface  of  the 
arch  which  renders  it  straight,  apparently  begins  in 
the  basal  leg  or  that  which  is  united  to  the  radicle ; 
for  this  leg,  as  we  often  observed,  is  first  bowed  back- 
wards from  the  other  leg.  This  movement  facilitates 
the  withdrawal  of  the  tip  of  the  epicotyl  or  of  the 
cotyledons,  as  the  case  may  be,  from  within  the  seed- 
coats  and  from  the  ground.  But  the  cotyledons  often 
emerge  from  the  ground  still  tightly  enclosed  within 
the  seed-coats,  which  apparently  serve  to  protect  them. 
The  seed-coats  are  afterwards  ruptured  and  cast  off  by 
the  swelling  of  the  closely  conjoined  cotyledons,  and  not 
by  any  movement  or  their  separation  from  one  another. 

Nevertheless,  in  some  few  cases,  especially  with  the 


102  RUPTURE   OF   THE   SEED-COATS.         CHAP.  H. 

Cucurbitaceae,  the  seed-coats  are  ruptured  by  a  curious 
contrivance,  described  by  M.  Flahault.*  A  heel  or 
peg  is  developed  on  one  side  of  the  summit  of  the 
radicle  or  base  of  the  hypocotyl  ;  and  this  holds  down 
the  lower  half  of  the  seed-coats  (the  radicle  being 
fixed  into  the  ground)  whilst  the  continued  growth  of 
the  arched  hypocotyl  forces  up- 
wards the  upper  half,  and  tears 
asunder  the  seed-coats  at  one  end, 
and  the  cotyledons  are  then  easily 
withdrawn.  The  accompanying 
figure  (Fig.  62)  will  render  this 
description  intelligible.  Forty- 
one  seeds  of  Cucurbita  ovifera 
were  laid  on  friable  peat  and  were 
covered  by  a  layer  about  an  inch 
in  thickness,  not  much  pressed 
down,  so  that  the  cotyledons  in 
being  dragged  up  were  subjected 
to  very  little  friction,  yet  forty  of 
them  came  up  naked,  the  seed- 
coats  being  left  buried  in  the  peat. 


heel  or  peg  projecting  This  Was  certainly  due  to  the  action 

on  one  side  from  summit  .     -                      „            ,            . 

of  radicle  and  holding  of  the  peg,  for  when  it  was  pre- 

down  lower  tip  of  seed-  vented  from  acting,  the  cotyledons, 

coats,  which  have  been  ..     ,.                      _ 

partially  ruptured  by  as   we   shall   presently  see,   were 

gronh  of  the  arched  lifte(j  U     gtm  enclosed   in   their 


seed-coats.  They  were,  however, 
cast  off  in  tne  course  of  two  or  three  days  by  the 
swelling  of  the  cotyledons.  Until  this  occurs  light  is 
excluded,  and  the  cotyledons  cannot  decompose  car- 
bonic acid  ;  but  no  one  probably  would  have  thought 
that  the  advantage  thus  gained  by  a  little  earlier  cast- 


*  •  Bull.  Soc.  Bot.  de  France,'  torn.  xxiv.  1877,  p.  201. 


CHAP.  II.        RUrTUEE  OF  THE  SEED-COATS.  103 

ing  off  of  the  seed-coats  would  be  sufficient  to  account 
for  the  development  of  the  peg.  Yet,  according  to 
M.  Flahault,  seedlings  which  have  been  prevented 
from  casting  their  seed-coats  whilst  beneath  the 
ground,  are  inferior  to  those  which  have  emerged  with 
their  cotyledons  naked  and  ready  to  act. 

The  peg  is  developed  with  extraordinary  rapidity ; 
for  it  could  only  just  be  distinguished  in  two  seed- 
lings, having  radicles  '35  inch  in  length,  but  after  an 
interval  of  only  24  hours  was  well  developed  in 
both.  It  is  formed,  according  to  Flahault,  by  the 
enlargement  of  the  layers  of  the  cortical  parenchyma 
at  the  base  of  the  hypocotyl.  If,  however,  we  judge 
by  the  effects  of  a  solution  of  permanganate  of 
potassium,  it  is  developed  on  the  exact  line  of 
junction  between  the  hypocotyl  and  radicle ;  for 
the  flat  lower  surface,  as  well  as  the  edges,  were 
coloured  brown  like  the  radicle:  whilst  the  upper 
slightly  inclined  surface  was  left  uncoloured  like  the 
hypocotyl,  excepting  indeed  in  one  out  of  33  im- 
mersed seedlings  in  which  a  large  part  of  the  upper  sur- 
face was  coloured  brown.  Secondary  roots  sometimes 
spring  from  the  lower  surface  of  the  peg,  which  thus 
seems  in  all  respects  to  partake  of  the  nature  of  the 
radicle.  The  peg  is  always  developed  on  the  side  which 
becomes  concave  by  the  arching  of  the  hypocotyl ; 
and  it  would  be  of  no  service  if  it  were  formed  on  any 
other  side.  It  is  also  always  developed  with  the  flat 
lower  side,  which,  as  just  stated,  forms  a  part  of  the 
radicle,  at  right  angles  to  it,  and  in  a  horizontal  plane. 
This  fact  was  clearly  shown  by  burying  some  of  the 
thin  flat  seeds  in  the  same  position  as  in  Fig.  62, 
excepting  that  they  were  not  laid  on  their  flat  broad 
sides,  but  with  one  edge  downwards.  Nine  seeds 
were  thus  planted,  and  the  peg  was  developed  in  the 


104  RUPTURE   OF   THE   SEED-COATS.         CHAP.  H. 

same  position,  relatively  to  the  radicle,  as  in  the 
figure ;  consequently  it  did  not  rest  on  the  flat  tip 
of  the  lower  half  of  the  seed-coats,  but  was  inserted 
like  a  wedge  between  the  two  tips.  As  the  arched 
hypocotyl  grew  upwards  it  tended  to  draw  up  the 
whole  seed,  and  the  peg  necessarily  rubbed  against 
both  tips,  but  did  not  hold  either  down.  The  result 
was,  that  the  cotyledons  of  five  out  of  the  nine  seeds 
thus  placed  were  raised  above  the  ground  still  enclosed 
within  their  seed-coats.  Four  seeds  were  buried  with 
the  end  from  which  the  radicle  protrudes  pointing 
vertically  downwards,  and  owing  to  the  peg  being 
always  developed  in  the  same  position,  its  apex  alone 
came  into  contact  with,  and  rubbed  against  the  tip  on 
one  side ;  the  result  was,  that  the  cotyledons  of  all 
four  emerged  still  within  their  seed-coats.  These  cases 
show  us  how  the  peg  acts  in  co-ordination  with  the 
position  which  the  flat,  thin,  broad  seeds  would  almost 
always  occupy  when  naturally  sown.  When  the  tip 
of  the  lower  half  of  the  seed-coats  was  cut  off,  Flahault 
found  (as  we  did  likewise)  that  the  peg  could  not  act, 
since  it  had  nothing  to  press  on,  and  the  cotyledons 
were  raised  above  the  ground  with  their  seed-coats  not 
cast  off.  Lastly,  nature  shows  us  the  use  of  the  peg  ; 
for  in  the  one  Cucurbitaceous  genus  known  to  us,  in 
which  the  cotyledons  are  hypogean  and  do  not  cast 
their  seed-coats,  namely,  Megarrhiza,  there  is  no 
vestige  of  a  peg.  This  structure  seems  to  be  present 
in  most  of  the  other  genera  in  the  family,  judging  from 
Flahault's  statements ;  we  found  it  well-developed  and 
properly  acting  in  Trichosanthes  anguina,  in  which  we 
hardly  expected  to  find  it,  as  the  cotyledons  are  some- 
what thick  and  fleshy.  Few  cases  can  be  advanced 
of  a  structure  better  adapted  for  a  special  purpose 
than  the  present  one. 


CHAP.  II.         RUPTURE   OF   THE   SEED-COATS.  105 

With  Mimosa  pudica  the  radicle  protrudes  from  a 
small  hole  in  the  sharp  edge  of  the  seed ;  and  on  its 
summit,  where  united  with  the  hypocotyl,  a  transverse 
ridge  is  developed  at  an  early  age,  which  clearly  aids 
in  splitting  the  tough  seed-coats ;  but  it  does  not  aid 
in  casting  them  off,  as  this  is  subsequently  effected  by 
the  swelling  of  the  cotyledons  after  they  have  been 
raised  above  the  ground.  The  ridge  or  heel  therefore 
acts  rather  differently  from  that  of  Cucurbita.  Its 
lower  surface  and  the  edges  were  coloured  brown  by 
the  permanganate  of  potassium,  but  not  the  upper 
surface.  It  is  a  singular  fact  that  after  the  ridge  has 
done  its  work  and  has  escaped  from  the  seed-coats, 
it  is  developed  into  a  frill  all  round  the  summit  of  the 
radicle.* 

At  the  base  of  the  enlarged  hypocotyl  of  Abronia 
umbellata,  where  it  blends  into  the  radicle,  there  is  a 
projection  or  heel  which  varies  in  shape,  but  its  out- 
line is  too  angular  in  our  former  figure  (Fig.  61).  The 
radicle  first  protrudes  from  a  small  hole  at  one  end  of 
the  tough,  leathery,  winged  fruit.  At  this  period  the 
upper  part  of  the  radicle  is  packed  within  the  fruit 
parallel  to  the  hypocotyl,  and  the  single  cotyledon  is 
doubled  back  parallel  to  the  latter.  The  swelling  of 
these  three  parts,  and  especially  the  rapid  development 
of  the  thick  heel  between  the  hypocotyl  and  radicle 
at  the  point  where  they  are  doubled,  ruptures  the 
tough  fruit  at  the  upper  end  and  allows  the  arched 
hypocotyl  to  emerge ;  and  this  seems  to  be  the  function 
of  the  heel.  A  seed  was  cut  out  of  the  fruit  and 


*  Our   attention  was  called  to  at  the  junction  of  the  radicle  and 

this  case  by  a  brief  statement  by  hypocotyl.     This  seed  possesses  a 

Nobbe    in    his    '  Handbuch  der  very  hard  and  tough  coat,  and 

Samenkunde,'  1876.  p.  215,  where  would  be  likely  to  require  aid  in 

a  figure  is  also  given  of  a  seedling  bursting  and  freeing  the  cotyle- 

of  Marty nia  with  a  heel  or  ridge  dons. 


106  RUPTUKE   OF   THE   SEED-COATS.         CHAP.  II. 

allowed  to  germinate  in  damp  air,  and  now  a  thin 
flat  disc  was  developed  all  round  the  base  of  the 
hypocotyl  and  grew  to  an  extraordinary  breadth,  like 
the  frill  described  under  Mimosa,  but  somewhat  broader. 
Flahault  says  that  with  Mirabilis,  a  member  of  the 
same  family  with  Abronia,  a  heel  or  collar  is  developed 
all  round  the  base  of  the  hypocotyl,  but  more  on  one 
side  than  on  the  other;  and  that  it  frees  the  coty- 
ledons from  their  seed-coats.  We  observed  only  old 
seeds,  and  these  were  ruptured  by  the  absorption  of 
moisture,  independently  of  any  aid  from  the  heel  and 
before  the  protrusion  of  the  radicle ;  but  it  does  not 
follow  from  our  experience  that  fresh  and  tough  fruits 
would  behave  in  a  like  manner. 

In  concluding  this  section  of  the  present  chapter  it 
may  be  convenient  to  summarise,  under  the  form  of  an 
illustration,  the  usual  movements  of  the  hypocotyls 
and  epicotyls  of  seedlings,  whilst  breaking  through  the 
ground  and  immediately  afterwards.  We  may  suppose 
a  man  to  be  thrown  down  on  his  hands  and  knees,  and 
at  the  same  time  to  one  side,  by  a  load  of  hay  falling 
on  him.  He  would  first  endeavour  to  get  his  arched 
back  upright,  wriggling  at  the  same  time  in  all 
directions  to  free  himself  a  little  from  the  surrounding 
pressure ;  and  this  may  represent  the  combined  effects 
of  apogeotropism  and  circumnutation,  when  a  seed  is  so 
buried  that  the  arched  hypocotyl  or  epicotyl  protrudes 
at  first  in  a  horizontal  or  inclined  plane.  The  man, 
still  wriggling,  would  then  raise  his  arched  back  as 
high  as  he  could ;  and  this  may  represent  the  growth 
and  continued  circumnutation  of  an  arched  hypocotyl 
or  epicotyl,  before  it  has  reached  the  surface  of  the 
ground.  As  soon  as  the  man  felt  himself  at  all  free,  he 
would  raise  the  upper  part  of  his  body,  whilst  still  on 


CHAP.  II.   CIRCUMNUTATION  OF  HYPOCOTYLS,  ETC.    107 

his  knees  and  still  wriggling ;  and  this  may  represent 
the  bowing  backwards  of  the  basal  leg  of  the  arch, 
which  in  most  cases  aids  in  the  withdrawal  of  the 
cotyledons  from  the  buried  and  ruptured  seed-coats, 
and  the  subsequent  straightening  of  the  whole  hypo- 
cotyl  or  epicotyl — circumnutation  still  continuing. 

Circumnutation  of  Hypocotyls  and  Epicotyls,  when 
erect. — The  hypocotyls,  epicotyls,  and  first  shoots  of  the 
many  seedlings  observed  by  us,  after  they  had  become 
straight  and  erect,  circumnutated  continuously.  The 
diversified  figures  described  by  them,  often  during  two 
successive  days,  have  been  shown  in  the  woodcuts  in 
the  last  chapter.  It  should  be  recollected  that  the 
dots  were  joined  by  straight  lines,  so  that  the  figures 
are  angular ;  but  if  the  observations  had  been  made 
every  few  minutes  the  lines  would  have  been  more 
or  less  curvilinear,  and  irregular  ellipses  or  ovals,  or 
perhaps  occasionally  circles,  would  have  been  formed. 
The  direction  of  the  longer  axes  of  the  ellipses  made 
during  the  same  day  or  on  successive  days  generally 
changed  completely,  so  as  to  stand  at  right  angles  to 
one  another.  The  number  of  irregular  ellipses  or 
circles  made  within  a  given  time  differs  much  with 
different  species.  Thus  with  Brassica  oleracea,  Cerintlie 
major,  and  Cucurbita  ovifera  about  four  such  figures 
were  completed  in  12  h. ;  whereas  with  Solanum  palina- 
canihum  arid  Opuntia  basilaris,  scarcely  more  than  one. 
The  figures  likewise  differ  greatly  in  size ;  thus  they 
were  very  small  and  in  some  degree  doubtful  in  Stapelia, 
and  large  in  Brassica,  &c.  The  ellipses  described  by 
Lathyrus  nissolia  and  Brassica  were  narrow,  whilst 
those  made  by  the  Oak  were  broad.  The  figures  are 
often  complicated  by  small  loops  and  zigzag  lines. 

As  most  seedling  plants  before  the  development 
of  true  leaves  are  of  low,  sometimes  very  low  stature, 


108    CIKCUMNUTATION  OF  HYPOCOTYLS,  ETC.    CHAP.  II. 

the  extreme  amount  of  movement  from  side  to  side 
of  their  circumnutating  stems  was  small ;  that  of 
the  hypocotyl  of  Githago  segetum  was  about  *2  of  an 
inch,  and  that  of  Cucuroita  ovifera  about  *28.  A 
very  young  shoot  of  Lathy rus  nissolia  moved  about 
•14,  that  of  an  American  oak  *2,  that  of  the  common 
nut  only  '04,  and  a  rather  tall  shoot  of  the  Asparagus 
•11  of  an  inch.  The  extreme  amount  of  movement 
of  the  sheath-like  cotyledon  of  Phalaris  Canariensis 
was  *3  of  an  inch ;  but  it  did  not  move  very  quickly, 
the  tip  crossing  on  one  occasion  five  divisions  of  the 
micrometer,  that  is,  yj^th  of  an  inch,  in  22  m.  5  s.  A 
seedling  Nolana  prostrata  travelled  the  same  distance 
in  10  m.  38  s.  Seedling  cabbages  circumutated  much 
more  quickly,  for  the  tip  of  a  cotyledon  crossed 
rJ0-th  of  an  inch  on  the  micrometer  in  3  m.  20  s. ;  and 
this  rapid  movement,  accompanied  by  incessant  oscil- 
lations, was  a  wonderful  spectacle  when  beheld  under 
the  microscope. 

The  absence  of  light,  for  at  least  a  day,  does  not 
interfere  in  the  least  with  the  circumnutation  of  the 
hypocotyls,  epicotyls,  or  young  shoots  of  the  various 
dicotyledonous  seedlings  observed  by  us  ;  nor  with  that 
of  the  young  shoots  of  some  monocotyledons.  The 
circumnutation  was  indeed  much  plainer  in  darkness 
than  in  light,  for  if  the  light  was  at  all  lateral -the 
stem  bent  towards  it  in  a  more  or  less  zigzag  course. 

Finally,  the  hypocotyls  of  many  seedlings  are  drawn 
during  the  winter  into  the  ground,  or  even  beneath  it 
so  that  they  disappear.  This  remarkable  process, 
which  apparently  serves  for  their  protection,  has 
been  fully  described  by  De  Yries.*  He  shows  that 


*  <Bot.  Zeitung/  1879,  p.  649.  burg,'  Jahrg.xvi.  p.  16,  as  quoted 
See  also  Winkler  in  '  Verhandl.  by  Haberlandt, '  Schutzeinrichun- 
des  Bot  Vereins  der  P.  Branden-  gen  der  Keimpflanze,'  1877,  p.  52. 


CHAP.  II.       CIKCUMNUTATION   OF   COTYLEDONS.         109 

it  is  effected  by  the  contraction  of  the  parenchyma- 
cells  of  the  root.  But  the  hypocotyl  itself  in  some 
cases  contracts  greatly,  and  although  at  first  smooth 
becomes  covered  with  zigzag  ridges,  as  we  observed 
with  Githago  segetum.  How  much  of  the  drawing 
down  and  burying  of  the  hypocotyl  of  Opuntia  lasilaris 
was  due  to  the  contraction  of  this  part  and  how  much 
to  that  of  the  radicle,  we  did  not  observe. 

Circumnutation  of  Cotyledons. — With  all  the  dico- 
tyledonous seedlings  described  in  the  last  chapter,  the 
cotyledons  were  in  constant  movement,  chiefly  in  a  ver- 
tical plane,  and  commonly  once  up  and  once  down  in 
the  course  of  the  24  hours.  But  there  were  many  excep- 
tions to  such  simplicity  of  movement ;  thus  the  cotyle- 
dons of  Ipomoea  caerulea  moved  13  times  either  upwards 
or  downwards  in  the  course  of  16  h.  18  m.  Those  of 
Oxalis  rosea  moved  in  the  same  manner  7  times  in  the 
course  of  24  h. ;  and  those  of  Cassia  tora  described  5 
irregular  ellipses  in  9  h.  The  cotyledons  of  some 
individuals  of  Mimosa  pudica  and  of  Lotus  Jacob&us 
moved  only  once  up  and  down  in  24  h.,  whilst  those  of 
others  performed  within  the  same  period  an  additional 
small  oscillation.  Thus  with  different  species,  and 
with  different  individuals  of  the  same  species,  there 
were  many  gradations  from  a  single  diurnal  move- 
ment to  oscillations  as  complex  as  those  of  the 
Ipomoaa  and  Cassia.  The  opposite  cotyledons  on  the 
same  seedling  move  to  a  certain  extent  independently 
of  one  another.  This  was  conspicuous  with  those  of 
Oxalis  sensitiva,  in  which  one  cotyledon  might  be 
seen  during  the  daytime  rising  up  until  it  stood 
vertically,  whilst  the  opposite  one  was  sinking  down. 

Although  the  movements  of  cotyledons  were  gene- 
rally in  nearly  the  same  vertical  plane,  yet  their 
upward  and  downward  courses  never  exactly  coin- 


110         CIRCUMNUTATION  OF  COTYLEDONS.       CHAP.  II. 

cided ;  so  that  ellipses,  more  or  less  narrow,  were 
described,  and  the  cotyledons  may  safely  be  said  to 
have  circumnutated.  Nor  could  this  fact  be  accounted 
for  by  the  mere  increase  in  length  of  the  cotyledons 
through  growth,  for  this  by  itself  would  not  induce 
any  lateral  movement.  That  there  was  lateral  move- 
ment in  some  instances,  as  with  the  cotyledons  of  the 
cabbage,  was  evident;  for  these,  besides  moving  up 
and  down,  changed  their  course  from  right  to  left  12 
times  in  14  h.  15  m.  With  Solanum  lycopersicum  the 
cotyledons,  after  falling  in  the  forenoon,  zigzagged 
from  side  to  side  between  12  and  4  P.M.,  and  then 
commenced  rising.  The  cotyledons  of  Lupinus  luteus 
are  so  thick  (about  '08  of  an  inch)  and  fleshy,*  that 
they  seemed  little  likely  to  move,  and  were  there- 
fore observed  with  especial  interest;  they  certainly 
moved  largely  up  and  down,  and  as  the  line  traced  was 
zigzag  there  was  some  lateral  movement.  The  nine 
cotyledons  of  a  seedling  Pinus  pinaster  plainly  circum- 
nutated ;  and  the  figures  described  approached  more 
nearly  to  irregular  circles  than  to  irregular  ovals  or 
ellipses.  The  sheath-like  cotyledons  of  the  Gra- 
mineaB  circumnutate,  that  is,  move  to  all  sides,  as 
plainly  as  do  the  hypocotyls  or  epicotyls  of  any  dico- 
tyledonous plants.  Lastly,  the  very  young  fronds  of 
a  Fern  and  of  a  Selaginella  circumnutated. 

In  a  large  majority  of  the  cases  which  were  care- 
fully observed,  the  cotyledons  sink  a  little  downwards 
in  the  forenoon,  and  rise  a  little  in  the  afternoon  or 
evening.  They  thus  stand  rather  more  highly  inclined 
during  the  night  than  during  the  mid-day,  at  which 

*  The  cotyledons,  though  bright  &c ,  1877,  p.  95),  on  the  gradations 
green,  resemble  to  a  certain  ex-  in  the  Leguminosae  between  sub- 
tent  hypogean  ones;  see  the  in-  aerial  and    subterranean  cotyle- 
teresting    discussion    by  Haber-  dons, 
landt  ('Die  Schutzeinrichtungen,' 


CAHP.  II.      CIRCUMNUTATION  OF   COTYLEDONS.          Ill 

time  they  are  expanded  almost  horizontally.  The 
circumnutating  movement  is  thus  at  least  partially 
periodic,  no  doubt  in  connection,  as  we  shall  hereafter 
see,  with  the  daily  alternations  of  light  and  darkness. 
The  cotyledons  of  several  plants  move  up  so  much  at 
night  as  to  stand  nearly  or  quite  vertically;  and  in 
this  latter  case  they  come  into  close  contact  with  one 
another.  On  the  other  hand,  the  cotyledons  of  a 
few  plants  sink  almost  or  quite  vertically  down  at 
night ;  and  in  this  latter  case  they  clasp  the  upper 
part  of  the  hypocotyl.  In  the  same  genus  Oxalis  the 
cotyledons  of  certain  species  stand  vertically  up,  and 
those  of  other  species  vertically  down,  at  night.  In 
all  such  cases  the  cotyledons  may  be  said  to  sleep, 
for  they  act  in  the  same  manner  as  do  the  leaves  of 
many  sleeping  plants.  This  is  a  movement  for  a 
special  purpose,  and  will  therefore  be  considered  in  a 
future  chapter  devoted  to  this  subject. 

In  order  to  gain  some  rude  notion  of  the  proportional 
number  of  cases .  in  which  the  cotyledons  of  dico- 
tyledonous plants  (hypogean  ones  being  of  course 
excluded)  changed  their  position  in  a  conspicuous 
manner  at  night,  one  or  more  species  in  several 
genera  were  cursorily  observed,  besides  those  described 
in  the  last  chapter.  Altogether  153  genera,  included 
in  as  many  families  as  could  be  procured,  were  thus 
observed  by  us.  The  cotyledons  were  looked  at  in 
the  middle  of  the  day  and  again  at  night ;  and  those 
were  noted  as  sleeping  which  stood  either  vertically 
or  at  an  angle  of  at  least  60°  above  or  beneath  the 
horizon.  Of  such  genera  there  were  26  ;  and  in  21  of 
them  the  cotyledons  of  some  of  the  species  rose,  and 
in  only  6  sank  at  night ;  and  some  of  these  latter 
cases  are  rather  doubtful  from  causes  to  be  explained 
in  the  chapter  on  the  sleep  of  cotyledons.  When 


112  PULVINI  OF   COTYLEDONS.  CHAP.  II. 

cotyledons  which  at  noon  were  nearly  horizontal,  stood 
at  night  at  more  than  20°  and  less  than  60°  above  the 
horizon,  they  were  recorded  as  "  plainly  raised ;"  and 
of  such  genera  there  were  38.  We  did  not  meet  with 
any  distinct  instances  of  cotyledons  periodically  sink- 
ing only  a  few  degrees  at  night,  although  no  doubt 
such  occur.  We  have  now  accounted  for  64  genera 
out  of  the  153,  and  there  remain  89  in  which  the 
cotyledons  did  not  change  their  position  at  night  by 
as  much  as  20° — that  is,  in  a  conspicuous  manner 
which  could  easily  be  detected  by  the  unaided  eye  and 
by  memory ;  but  it  must  not  be  inferred  from  this 
statement  that  these  cotyledons  did  not  move  at  all, 
for  in  several  cases  a  rise  of  a  few  degrees  was  re- 
corded, when  they  were  carefully  observed.  The 
number  89  might  have  been  a  little  increased,  for  the 
cotyledons  remained  almost  horizontal  at  night  in 
some  species  in  a  few  genera,  for  instance,  Trifo- 
lium  and  Geranium,  which  are  included  amongst  the 
sleepers,  such  genera  might  therefore  have  been  added 
to  the  89.  Again,  one  species  of  Oxalis  generally 
raised  its  cotyledons  at  night  more  than  20°  and  less 
than  60°  above  the  horizon  ;  so  that  this  genus  might 
have  been  included  under  two  heads.  But  as  several 
species  in  the  same  genus  were  not  often  observed, 
such  double  entries  have  been  avoided. 

In  a  future  chapter  it  will  be  shown  that  the  leaves 
of  many  plants  which  do  not  sleep,  rise  a  few  degrees  in 
the  evening  and  during  the  early  part  of  the  night ; 
and  it  will  be  convenient  to  defer  until  then  the 
consideration  of  the  periodicity  of  the  movements  of 
cotyledons. 

On  the  Pulvini  or  Joints  of  Cotyledons. — With  several 
of  the  seedlings  described  in  this  and  the  last  chapter, 
the  summit  of  the  petiole  is  developed  into  a  pulvinus. 


CHAP.  II. 


PULVINI  OF  COTYLEDONS. 


113 


cushion,  or  joint  (as  this  organ  has  been  variously 
called),  like  that  with  which  many  leaves  are  provided. 
It  consists  of  a  mass  of  small  cells  usually  of  a  pale 
colour  from  the  absence  of  chlorophyll,  and  with  its 
outline  more  or  less  convex,  as  shown  in  the  annexed 
figure.  In  the  case  of  Oxalis 
sensitiva  two-thirds  of  the 
petiole,  and  in  that  of  Mi- 
mosa pudica,  apparently  the 
whole  of  the  short  sub- 
petioles  of  the  leaflets  have 
been  converted  into  pulvini. 
With  pulvinated  leaves  (i.e. 
those  provided  with  a  pul- 
vinus) their  periodical  move- 
ments depend,  according  to 
Pfeffer,*  on  the  cells  of  the 
pulvinus  alternately  expand- 
ing more  quickly  on  one  side 
than  on  the  other;  whereas 
the  similar  movements  of 
leaves  not  provided  with  pul- 
vini, depend  on  their  growth  Oxalis  rosea:  longitudinal  section 

of  a  pulvinus  on  the  summit 


of  the  petiole  of  a  cotyledon, 
drawn  with  the  camera  lucida, 
magnified  75  times :  p,  />,  pe- 
tiole ;  /,  fibro- vascular  bundle  ; 
6, 6,  commencement  of  blade  ol 
cotyledon. 


being  alternately  more  rapid 
on  one  side  than  on  the 
other.f  As  long  as  a  leaf 
provided  with  a  pulvinus  is 
young  and  continues  to  grow, 
its  movement  depends  on  both  these  causes  combined  ;J 
and  if  the  view  now  held  by  many  botanists  be  sound, 
namely,  that  growth  is  always  preceded  by  the  expan- 
sion of  the  growing  cells,  then  the  difference  between 
the  movements  induced  by  the  aid  of  pulvini  and 


*  'Die   Periodische   Bewegun- 
gen  der  Blattorgane,'  1875. 


t  Batalin,  'Flora,'  Oct.  1st,  1873 . 
I  Pfeffer,  ibid.  p.  5. 


114  PULVINI  OF   COTYLEDONS.  CHAP.  II. 

without  such  aid,  is  reduced  to  the  expansion  of  the 
cells  not  being  followed  by  growth  in  the  first  case, 
and  being  so  followed  in  the  second  case. 

Dots  were  made  with  Indian  ink  along  the  midrib 
of  both  pulvinated  cotyledons  of  a  rather  old  seedling 
of  Oxalis  VdLdiviana ;  their  distances  were  repeatedly 
measured  with  an  eye-piece  micrometer  during  8|  days, 
and  they  did  not  exhibit  the  least  trace  of  increase. 
It  is  therefore  almost  certain  that  the  pulvinus  itself 
was  not  then  growing.  Nevertheless,  during  this 
whole  time  and  for  ten  days  afterwards,  these  coty- 
ledons rose  vertically  every  night.  In  the  case  of 
some  seedlings  raised  from  seeds  purchased  under  the 
name  of  Oxalis  floribunda,  the  cotyledons  continued 
for  a  long  time  to  move  vertically  down  at  night,  and 
the  movement  apparently  depended  exclusively  on 
the  pulvini,  for  their  petioles  were  of  nearly  the  same 
length  in  young,  and  in  old  seedlings  which  had  pro- 
duced true  leaves.  With  some  species  of  Cassia,  on 
the  other  hand,  it  was  obvious  without  any  measure- 
ment that  the  pulvinated  cotyledons  continued  to 
increase  greatly  in  length  during  some  weeks ;  so  that 
here  the  expansion  of  the  cells  of  the  pulvini  and  the 
growth  of  the  petiole  were  probably  combined  in 
causing  their  prolonged  periodic  movements.  It  was 
equally  evident  that  the  cotyledons  of  many  plants, 
not  provided  with  pulvini,  increased  rapidly  in  length ; 
and  their  periodic  movements  no  doubt  were  exclu- 
sively due  to  growth. 

In  accordance  with  the  view  that  the  periodic 
movements  of  all  cotyledons  depend  primarily  on  the 
expansion  of  the  cells,  whether  or  not  followed  by 
growth,  we  can  understand  the  fact  that  there  is  but 
little  difference  in  the  kind  or  form  of  movement 
in  the  two  sets  of  cases.  This  may  be  seen  by  com- 


CHAP.  II.  PULVINI  OF   COTYLEDONS.  115 

paring  the  diagrams  given  in  the  last  chapter.  Thus 
the  movements  of  the  cotyledons  of  Brassica  oleracea 
and  of  Ipomcea  cserulea,  which  are  not  provided  with 
pulvini,  are  as  complex  as  those  of  Oxalis  and  Cassia 
which  are  thus  provided.  The  pulvinated  cotyledons 
of  some  individuals  of  Mimosa  pudica  and  Lotus 
Jacobseus  made  only  a  single  oscillation,  whilst  those 
of  other  individuals  moved  twice  up  and  down  in  the 
course  of  24  hours ;  so  it  was  occasionally  with  the 
cotyledons  of  Cueurbita  ovifera,  which  are  destitute  of 
a  pulvinus.  The  movements  of  pulvinated  cotyledons 
are  generally  larger  in  extent  than  those  without  a 
pulvinus;  nevertheless  some  of  the  latter  moved 
through  an  angle  of  90°.  There  is,  however,  one 
important  difference  in  the  two  sets  of  cases;  the 
nocturnal  movements  of  cotyledons  without  pulvini, 
for  instance,  those  in  the  Cruciferse,  Cucurbitacese, 
Githago,  and  Beta,  never  last  even  for  a  week,  to  any 
conspicuous  degree.  Pulvinated  cotyledons,  on  the 
other  hand,  continue  to  rise  at  night  for  a  much 
longer  period,  even  for  more  than  a  month,  as  we 
shall  now  show.  But  the  period  no  doubt  depends 
largely  on  the  temperature  to  which  the  seedlings  are 
exposed  and  their  consequent  rate  of  development. 

Oxalis  Valdiviana. — Some  cotyledons  which  had  lately  opened 
and  were  horizontal  on  March  6th  at  noon,  stood  at  night  ver- 
tically up ;  on  the  13th  the  first  true  leaf  was  formed,  and  was 
embraced  at  night  by  the  cotyledons ;  on  April  9th,  after  an  in- 
terval of  35  days,  six  leaves  were  developed,  and  yet  the  coty- 
ledons rose  almost  vertically  at  night.  The  cotyledons  of 
another  seedling,  which  when  first  observed  had  already  pro- 
duced a  leaf,  stood  vertically  at  night  and  continued  to  do  so  for 
11  additional  days.  After  16  days  from  the  first  observation 
two  leaves  were  developed,  and  the  cotyledons  were  still  greatly 
raised  at  night.  After  21  days  the  cotyledons  during  the  day 
were  deflected  beneath  the  horizon,  but  at  night  were  raised  45° 


116  PULVINI   OF  COTYLEDONS.  CHAP.  II. 

above  it.  After  24  days  from  the  first  observation  (begun  after 
a  true  leaf  had  been  developed)  the  cotyledons  ceased  to  rise  at 
night. 

Ozalis  (Biophytum)  sensitiva. — The  cotyledons  of  several  seed- 
lings, 45  days  after  their  first  expansion,  stood  nearly  vertical  at 
night,  and  closely  embraced  either  one  or  two  true  leaves  which 
by  this  time  had  been  formed.  These  seedlings  had  been  kept 
in  a  very  warm  house,  and  their  development  had  been  rapid. 

Oxalis  corniculata.—Tlie  cotyledons  do  not  stand  vertical  at 
night,  but  generally  rise  to  an  angle  of  about  45°  above  the 
horizon.  They  continued  thus  to  act  for  23  days  after  their 
first  expansion,  by  which  time  two  leaves  had  been  formed ; 
even  after  29  days  they  still  rose  moderately  above  their  hori- 
zontal or  downwardly  deflected  diurnal  position. 

Mimosa  pudica. — The  cotyledons  were  expanded  for  the  first 
time  on  Nov.  2nd,  and  stood  vertical  at  night.  On  the  15th  the 
first  leaf  was  formed,  and  at  night  the  cotyledons  were  vertical. 
On  the  28th  they  behaved  in  the  same  manner.  On  Dec.  15th, 
that  is  after  44  days,  the  cotyledons  were  still  considerably 
raised  at  night;  but  those  of  another  seedling,  only  one  day 
older,  were  raised  very  little. 

Mimosa  albida.- — A  seedling  was  observed  during  only  12  days, 
by  which  time  a  leaf  had  been  formed,  and  the  cotyledons  were 
then  quite  vertical  at  night. 

Trifolium  subterraneum. — A  seedling,  8  days  old,  had  its  coty- 
ledons horizontal  at  10.30  A.M.  and  vertical  at  9.15  P.M.  After  an 
interval  of  two  months,  by  which  time  the  first  and  second  true 
leaves  had  been  developed,  the  cotyledons  still  performed  the 
same  movement.  They  had  now  increased  greatly  in  size,  and 
had  become  oval;  and  their  petioles  were  actually  *8  of  an  inch 
in  length ! 

Trifolium  strict um. — After  17  days  the  cotyledons  still  rose  at 
night,  but  were  not  afterwards  observed. 

Lotus  Jacobceus. — The  cotyledons  of  some  seedlings  having 
well-developed  leaves  rose  to  an  angle  of  about  45°  at  night; 
and  even  after  3  or  4  whorls  of  leaves  had  been  formed,  the  co- 
tyledons rose  at  night  considerably  above  their  diurnal  hori- 
zontal position. 

Cassia  mimosoides. — The  cotyledons  of  this  Indian  species, 
14  dajs  after  their  first  expansion,  and  when  a  leaf  had  been 
formed,  stood  during  the  day  horizontal,  and  at  night  vertical. 

Cassia  sp  ?  (a  large  S.  Brazilian  tree  raised  from  seeds  sent  us 


CHAP.  II.  PULVINI   OF   COTYLEDONS.  117 

by  F.  Miiller). — The  cotyledons,  after  16  days  from  their  first 
expansion,  had  increased  greatly  in  size  with  two  leaves  just 
formed.  They  stood  horizontally  during  the  day  and  vertically 
at  night,  but  were  not  afterwards  observed. 

Cassia  neghcta  (likewise  a  S.  Brazilian  species). — A  seedling, 
34  days  after  the  first  expansion  of  its  cotyledons,  was  between  3 
and  4  inches  in  height,  with  3  well-developed  leaves;  and  the 
cotyledons,  which  during  the  day  were  nearly  horizontal,  at  night 
stood  vertical,  closely  embracing  the  young  stem.  The  cotyle- 
dons of  another  seedling  of  the  same  age,  5  inches  in  height, 
with  4  well-developed  leaves,  behaved  at  night  in  exactly  the 
same  manner. 

It  is  known  *  that  there  is  no  difference  in  structure 
between  the  upper  and  lower  halves  of  the  pulvini  of 
leaves,  sufficient  to  account  for  their  upward  or  down- 
ward movements.  In  this  respect  cotyledons  offer  an 
unusally  good  opportunity  for  comparing  the  structure 
of  the  two  halves ;  for  the  cotyledons  of  Oxalis  Valdi- 
viana  rise  vertically  at  night,  whilst  those  of  0.  rosea 
sink  vertically ;  yet  when  sections  of  their  pulvini  were 
made,  no  clear  difference  could  be  detected  between  the 
corresponding  halves  of  this  organ  in  the  two  species 
which  move  so  differently.  With  0.  rosea,  however, 
there  were  rather  more  cells  in  the  lower  than  in  the 
upper  half,  but  this  was  likewise  the  case  in  one  speci- 
men of  0.  Valdiviana.  The  cotyledons  of  both  species 
(3J  mm.  in  length)  were  examined  in  the  morning 
whilst  extended  horizontally,  and  the  upper  surface  of 
the  pulvinus  of  0.  rosea  was  then  wrinkled  transversely, 
showing  that  it  was  in  a  state  of  compression,  and  this 
might  have  been  expected  as  the  cotyledons  sink  at 
night;  with  0.  Valdiviana  it  was  the  lower  surface 
which  was  wrinkled,  and  its  cotyledons  rise  at  night. 

Trifolium  is  a  natural  genus,  and  the  leaves  of  a]l 


*  Pfeffer,  '  Die  Period.  Bewegungen,'  1875,  p.  157. 


118  PULVINI  OF  COTYLEDONS.  CHAP.  IL 

the  species  seen  by  us  are  pulvinated ;  so  it  is  with 
the  cotyledons  of  T.  svMerraneum  and  strictum,  which 
stand  vertically  at  night ;  whereas  those  of  T.  resupi- 
natum  exhibit  not  a  trace  of  a  pulvinus,  nor  of  any 
nocturnal  movement.  This  was  ascertained  by  mea- 
suring the  distance  between  the  tips  of  the  cotyledons 
of  four  seedlings  at  mid-day  and  at  night.  In  this 
species,  however,  as  in  the  others,  the  first-formed  leaf, 
which  is  simple  or  not  trifoliate,  rises  up  and  sleeps 
like  the  terminal  leaflet  on  a  mature  plant. 

In  another  natural  genus,  Oxalis,  the  cotyledons  of 
0.  Valdiviana,  rosea, floribunda,  articulata,  and  sensitiva 
are  pulvinated,  and  all  move  at  night  into  an  upward 
or  downward  vertical  position.  In  these  several  species 
the  pulvinus  is  seated  close  to  the  blade  of  the  coty- 
ledon, as  is  the  usual  rule  with  most  plants.  Oxalis  cor- 
niculata  (var.  Atro-purpurea)  differs  in  several  respects ; 
the  cotyledons  rise  at  night  to  a  very  variable  amount, 
rarely  more  than  45°;  and  in  one  lot  of  seedlings 
(purchased  under  the  name  of  0.  tropasoloides,  but 
certainly  .belonging  to  the  above  variety)  they  rose 
only  from  5°  to  15°  above  the  horizon.  The  pulvinus 
is  developed  imperfectly  and  to  an  extremely  variable 
degree,  so  that  apparently  it  is  tending  towards  abor- 
tion. No  such  case  has  hitherto,  we  believe,  been 
described.  It  is  coloured  green  from  its  cells  con- 
taining chlorophyll;  and  it  is  seated  nearly  in  the 
middle  of  the  petiole,  instead  of  at  the  upper  end  as 
in  all  the  other  species.  The  nocturnal  movement  is 
effected  partly  by  its  aid,  and  partly  by  the  growth  of 
the  upper  part  of  the  petiole  as  in  the  case  of  plants 
destitute  of  a  pulvinus.  From  these  several  reasons 
and  from  our  havingt  partially  traced  the  develop- 
ment of  the  pulvinus  from  an  early  age,  the  case 
seems  worth  describing  in  some  detail. 


CHAP.  II. 


PULVINI  OF   COTYLEDONS. 


119 


When  the  cotyledons  of  0.  corniculata  were  dissected  out  of  a 
seed  from  which  they  would  soon  have  naturally  emerged,  no 
trace  of  a  pulvinus  could  be  detected ;  and  all  the  cells  forming 
the  short  petiole,  7  in  number  in  a  longitudinal  row,  were  of  nearly 
equal  size.  In  seedlings  one  or  two  days  old,  the  pulvinus  was 
so  indistinct  that  we  thought  at  first  that  it  did  not  exist ;  but 
in  the  middle  of  the  petiole  an  ill-defined  transverse  zone  of  cells 
could  be  seen,  which  were  much  shorter  than  those  both  above 
and  below,  although  of  the  same  breadth  with  them.  They 
presented  the  appearance  of  having  been  just  formed  by  the 
transverse  division  of  longer  cells ;  and  there  can  be  little  doubt 
that  this  had  occurred,  for  the  cells  in  the  petiole  which  had 

Fig.  64. 


A.  B. 

Oxalis  corniculata :  A  and  B  the  almost  rudimentary  pulvini  of  the  coty- 
ledons of  two  rather  old  seedlings,  viewed  as  transparent  objects. 
Magnified  50  times. 

been  dissected  out  of  the  seed  averaged  in  length  7  divisions 
of  the  micrometer  (each  division  equalling  '003  mm  ),  and  were 
a  little  longer  than  those  forming  a  well-developed  pulvinus, 
which  varied  between  4  and  6  of  these  same  divisions.  After  a 
few  additional  days  the  ill-defined  zone  of  cells  becomes  distinct, 
and  although  it  does  not  extend  across  the  whole  width  of  the 
petiole,  and  although  the  cells  are  of  a  green  colour  from  contain- 
ing chlorophyll,  yet  they  certainly  constitute  a  pulvinus,  which, 
as  we  shall  presently  see,  acts  as  one.  These  small  cells  were 
arranged  in  longitudinal  rows,  and  varied  from  4  to  7  in  number ; 
and  the  cells  themselves  varied  in  length  in  different  parts  of  the 


120  PULVINI   OF  COTYLEDONS.  CHAP.  II. 

same  pulvinus  and  in  different  individuals.  In  the  accompany- 
ing figures,  A  and  B  (Fig.  64),  we  have  views  of  the  epidermis  * 
in  the  middle  part  of  the  petioles  of  two  seedlings,  in  which  the 
pulvinus  was  for  this  species  well  developed.  They  offer  a 
striking  contrast  with  the  pulvinus  of  0.  rosea  (see  former 
Fig.  63),  or  of  0.  Valdiviana.  With  the  seedlings,  falsely  called 
0.  tropceoloides,  the  cotyledons  of  which  rise  very  little  at  night, 
the  small  cells  were  still  fewer  in  number  and  in  parts  formed 
a  single  transverse  row,  and  in  other  parts  short  longitudinal 
rows  of  only  two  or  three.  Nevertheless  they  sufficed  to  attract 
the  eye,  when  the  whole  petiole  was  viewed  as  a  transparent 
object  beneath  the  microscope.  In  these  seedlings  there  could 
hardly  be  a  doubt  that  the  pulvinus  was  becoming  rudimentary 
and  tending  to  disappear;  and  this  accounts  for  its  great 
variability  in  structure  and  function. 

In  the  following  Table  some  measurements  of  the  cells  in 
fairly  well-developed  pulvini  of  0.  corniculata  are  given  : — 

Seedling  1  day  old,  with  cotyledon  2'3  mm.  in  length. 

Divisions  of 
Micrometer.f 

Average  length  of  cells  of  pulvinus         6  to  7 

Length  of  longest  cell  below  the  pulvinus      13 

Length  of  longest  cell  above  the  pulvinus      20 

Seedling  5  d  <ys  old,  cotyledon  3'1  mm.  in  length,  wi!h  the  pulvinus 
quite  distinct. 

Average  length  of  cells  of  pulvinus         6 

Length  of  longest  cell  below  the  pnlvinus      22 

Length  of  longest  cell  above  the  pulvinus      40 

Seedling  8  days  old,  cotyledon  5  mm.  in  length,  with  a  true  leaf 
formed  but  not  yet  expanded, 

Average  length  of  cells  of  pulvinus         9 

Length  of  longest  cell  below  the  pulvinus      44 

Length  of  longest  cell  above  the  pulvinus      70 

Seedling  13  days  old,  coti/ledon  4*5  mm.  in  length,  with  a  smill 
true  leaf  fully  developed. 

Average  length  of  cells  of  pulvinus         7 

Length  of  longest  cell  below  the  pulvinus      30 

Length  of  longest  cell  above  the  pulvinus      CO 


*  Longitudinal   sections    show  pulvinus. 

that  the  forms  of  the  epidermic  f  Each  division  equalled  '003 

cells  may  be  taken  as  a  fair  repre-  mm. 
sentation  of  those  constituting  the 


CHAP.  IE.  PULVINI  OF   COTYLEDONS.  121 

We  litre  see  that  the  cells  of  the  pulvinus  increase  but  little 
in  length  with  advancing  age,  in  comparison  with  those  of  the 
petiole  both  above  and  below  it ;  but  they  continue  to  grow  in 
width,  and  keep  equal  in  this  respect  with  the  other  cells  of 
the  petiole.  The  rate  of  growth,  however,  varies  in  all  parts 
of  the  cotyledons,  as  may  be  observed  in  the  measurements  of 
the  8-days'  old  seedling. 

The  cotyledons  of  seedlings  only  a  day  old  rise  at  night  con- 
siderably, sometimes  as  much  as  afterwards;  but  there  was 
much  variation  in  this  respect.  As  the  pulvinus  is  so  indistinct 
at  first,  the  movement  probably  does  not  then  depend  on  the 
expansion  of  its  cells,  but  on  periodically  unequal  growth  in 
the  petiole.  By  the  comparison  of  seedlings  of  different  known 
ages,  it  was  evident  that  the  chief  seat  of  growth  of  the  petiole 
was  in  the  upper  part  between  the  pulvinus  and  the  blade; 
and  this  agrees  with  the  fact  (shown  in  the  measurements  above 
given)  that  the  cells  grow  to  a  greater  length  in  the  upper  than 
in  the  lower  part.  With  a  seedling  1 1  days  old,  the  nocturnal 
rise  was  found  to  depend  largely  on  the  action  of  the  pulvinus, 
for  the  petiole  at  night  was  curved  upwards  at  this  point ;  and 
during  the  day,  whilst  the  petiole  was  horizontal,  the  lower 
surface  of  the  pulvinus  was  wrinkled  with  the  upper  surface 
tense.  Although  the  cotyledons  at  an  advanced  age  do  not  rise 
at  night  to  a  higher  inclination  than  whilst  young,  yet  they  have 
to  pass  through  a  larger  angle  (in  one  instance  amounting  to 
63°)  to  gain  their  nocturnal  position,  as  they  are  generally 
deflected  beneath  the  horizon  during  the  day.  Even  witk  the 
11-days'  old  seedling  the  movement  did  not  depend  exclusively 
on  the  pulvinus,  for  the  blade  where  joined  to  the  petiole  was 
curved  upwards,  and  this  must  be  attributed  to  unequal  growth. 
Therefore  the  periodic  movements  of  the  cotyledons  of  0.  corni- 
culata  depend  on  two  distinct  but  conjoint  actions,  namely,  the 
expansion  of  the  cells  of  the  pulvinus  and.  on  the  growth  of 
the  upper  part  of  the  petiole,  including  the  base  of  the  blade. 

Lotus  Jacobceus. — The  seedlings  of  this  plant  present  a  case 
parallel  to  that  of  Oxalis  corniculata  in  some  respects,  and  in 
others  unique,  as  far  as  we  have  seen.  The  cotyledons  during 
the  first  4  or  5  days  of  their  life  do  not  exhibit  any  plain  noc- 
turnal movement ;  but  afterwards  they  stand  vertically  or 
almost  vertically  up  at  night.  There  is,  however,  some  degree  of 
variability  in  this  respect,  apparently  dependent  on  the  season 
and  on  the  degree  to  which  they  have  been  illuminated  during 


122  PULVINI   OF   COTYLEDONS.  CHAP.  II. 

the  day.  With  older  seedlings,  having  cotyledons  4  mm.  in 
length,  which  rise  considerably  at  night,  there  is  a  well  -deve- 
loped pulvinus  close  to  the  blade,  colourless,  and  rather  nar- 
rower than  the  rest  of  the  petiole,  from  which  it  is  abruptly 
separated.  It  is  formed  of  a  mass  of  small  cells  of  an  average 
length  of  -021  mm. ;  whereas  the  cells  in  the  lower  part  of  the 
petiole  are  about  "06  mm.,  and  those  in  the  blade  from  -034  to 
•04  mm.  in  length.  The  epidermic  cells  in  the  lower  part  of  the 
petiole .  project  conically,  and  thus  differ  in  shape  from  those 
over  the  pulvinus. 

Turning  now  to  very  young  seedlings,  the  cotyledons  of  which 
do  not  rise  at  night  and  are  only  from  2  to  2£  mm.  in  length, 
their  petioles  do  not  exhibit  any  defined  zone  of  small  cells, 
destitute  of  chlorophyll  and  differing  in  shape  exteriorly  from 
the  lower  ones.  Nevertheless,  the  cells  at  the  place  where  a 
pulvinus  will  afterwards  be  developed  are  smaller  (being  on  an 
average  '015  mm.  in  length)  than  those  in  the  lower  parts  of 
the  same  petiole,  which  gradually  become  larger  in  proceeding 
downwards,  the  largest  being  -030  mm.  in  length.  At  this  early 
age  the  cells  of  the  blade  are  about  '027  mm.  in  length.  We 
thus  see  that  the  pulvinus  is  formed  by  the  cells  in  the  upper- 
most part  of  the  petiole,  continuing  for  only  a  short  time  to 
increase  in  length,  then  being  arrested  in  their  growth,  accom- 
panied by  the  loss  of  their  chlorophyll  grains ;  whilst  the  cells 
in  the  lower  part  of  the  petiole  continue  for  a  long  time  to 
increase  in  length,  those  of  the  epidermis  becoming  more  conical. 
The  singular  fact  of  the  cotyledons  of  this  plant  not  sleeping  at 
first  is  therefore  due  to  the  pulvinus  not  being  developed  at  an 
early  age. 

We  learn  from  these  two  cases  of  Lotus  and  Oxalis, 
that  the  development  of  a  pulvinus  follows  from  the 
growth  of  the  cells  over  a  small  defined  space  of  the 
petiole  being  almost  arrested  at  an  early  age.  With 
Lotus  Jacobseus  the  cells  at  first  increase  a  little  in 
length ;  in  Oxalis  corniculata  they  decrease  a  little, 
owing  to  self-division.  A  mass  of  such  small  cells 
forming  a  pulvinus,  might  therefore  be  either  acquired 
or  lost  without  any  special  difficulty,  by  different 
species  in  the  same  natural  genus :  and  we  know  that 


CHAP.  II.      DISTURBED  PERIODIC  MOVEMENTS.  123 

with  seedlings  of  Trifolium,  Lotus,  and  Oxalis  some  of 
the  species  have  a  well-developed  pulvinus,  and  others 
have  none,  or  one  in  a  rudimentary  condition.  As  the 
movements  caused  by  the  alternate  turgescence  of 
the  cells  in  the  two  halves  of  a  pulvinus,  must  be 
largely  determined  by  the  extensibility  and  subse- 
quent contraction  of  their  walls,  we  can  perhaps  under- 
stand why  a  large  number  of  small  cells  will  be  more 
efficient  than  a  small  number  of  large  cells  occupying 
the  same  space.  As  a  pulvinus  is  formed  by  the 
arrestment  of  the  growth  of  its  cells,  movements  de- 
pendent on  their  action  may  be  long-continued  without 
any  increase  in  length  of  the  part  thus  provided; 
and  such  long-continued  movements  seem  to  be  one 
chief  end  gained  by  the  development  of  a  pulvinus. 
Long-continued  movement  would  be  impossible  in  any 
part,  without  an  inordinate  increase  in  its  length,  if  the 
turgescence  of  the  cells  was  always  followed  by  growth. 
Disturbance  of  the  Periodic  Movements  of  Cotyledons  ly 
Light. — The  hypocotyls  and  cotyledons  of  most  seed- 
ling plants  are,  as  is  well  known,  extremely  heliotropic ; 
but  cotyledons,  besides  being  heliotropic,  are  affected 
paratonically  (to  use  Sachs'  expression)  by  light  p  that 
is,  their  daily  periodic  movements  are  greatly  and 
quickly  disturbed  by  changes  in  its  intensity  or  by 
its  absence.  It  is  not  that  they  cease  to  circumnutate 
in  darkness,  for  in  all  the  many  cases  observed  by  us 
they  continued  to  do  so;  but  the  normal  order  of 
their  movements  in  relation  to  the  alternations  of  day 
and  night  is  much  disturbed  or  quite  annulled.  This 
holds  good  with  species  the  cotyledons  of  which  rise 
or  sink  so  much  at  night  that  they  may  be  said  to 
sleep,  as  well  as  with  others  which  rise  only  a  little. 
But  different  species  are  affected  in  very  different 
degrees  by  changes  in  the  light. 


124          DISTURBED   PERIODIC  MOVEMENTS.      CHAP.  II. 

For  instance,  the  cotyledons  of  Beta  vulgaris,  Solanum  lycoper- 
sicum,  Cerinthe  major,  and  Lupinus  luteus,  when  placed  in  dark- 
ness, moved  down  during  the  afternoon  and  early  night,  instead 
of  rising  as  they  would  have  done  if  they  had  been  exposed  to 
the  light.  All  the  individuals  of  the  Solanum  did  not  behave 
in  the  same  manner,  for  the  cotyledons  of  one  circumnutated 
about  the  same  spot  between  2.30  and  10  P.M.  The  cotyledons 
of  a  seedling  of  Oxalis  corniculata,  which  was  feebly  illuminated 
from  above,  moved  downwards  during  the  first  morning  in  the 
normal  manner,  but  on  the  second  morning  it  moved  upwards. 
The  cotyledons  of  Lotus  Jacobceus  were  not  affected  by  4  h.  of 
complete  darkness,  but  when  placed  under  a  double  skylight 
and  thus  feebly  illuminated,  they  quite  lost  their  periodical 
movements  on  the  third  morning.  On  the  other  hand,  the 
cotyledons  of  Cucurbita  ovifera  moved  in  the  normal  manner 
during  a  whole  day  in  darkness. 

Seedlings  of  Githago  segetum  were  feebly  illuminated  from 
above  in  the  morning  before  their  cotyledons  had  expanded,  and 
they  remained  closed  for  the  next  40  h.  Other  seedlings  were 
placed  in  the  dark  after  their  cotyledons  had  opened  in  the 
morning  and  these  did  not  begin  to  close  until  about  4  h.  had 
elapsed.  The  cotyledons  of  Oxalis  rosea  sank  vertically  down- 
wards after  being  left  for  1  h.  20  m.  in  darkness ;  but  those  of 
some  other  species  of  Oxalis  were  not  affected  by  several  hours 
of  darkness.  The  cotyledons  of  several  species  of  Cassia  are 
eminently  susceptible  to  changes  in  the  degree  of  light  to  which 
they  are  exposed :  thus  seedlings  of  an  unnamed  S.  Brazilian 
species  (a  large  and  beautiful  tree)  were  brought  out  of  the  hot- 
house and  placed  on  a  table  in  the  middle  of  a  room  with  two 
north-east  and  one  north-west  window,  so  that  they  were  fairly 
well  illuminated,  though  of  course  less  so  than  in  the  hot-house, 
the  day  being  moderately  bright ;  and  after  36  m.  the  cotyledons 
which  had  been  horizontal  rose  up  vertically  and  closed  together 
as  when  asleep ;  after  thus  remaining  on  the  table  for  1  h.  13  m. 
they  began  to  open.  The  cotyledons  of  young  seedlings  of  another 
Brazilian  species  and  of  C.  neglecta,  treated  in  the  same  manner, 
behaved  similarly,  excepting  that  they  did  not  rise  up  quite  so 
much ;  they  again  became  horizontal  after  about  an  hour. 

Here  is  a  more  interesting  case :  seedlings  of  Cassia  tora  in 
two  pots,  which  had  stood  for  some  time  on  the  table  in  the 
room  just  described,  had  their  cotyledons  horizontal.  One  pot 
was  now  exposed  for  2  h.  to  dull  sunshine,  and  the  cotyledons 


CHAP.  IT.         SENSITIVENESS   OF  COTYLEDONS.  125 

remained  horizontal ;  it  was  then  brought  back  to  the  table,  and 
after  50m.  the  cotyledons  had  risen  68°  above  the  horizon. 
The  other  pot  was  placed  during  the  same  2  h.  behind  a  screen 
in  the  room,  where  the  light  was  very  obscure,  and  the  cotyledons 
rose  63°  above  the  horizon ;  the  pot  was  then  replaced  on  the 
table,  and  after  50  m.  the  cotyledons  had  fallen  33°.  These  two 
pots  with  seedlings  of  the  same  age  stood  close  together,  and 
were  exposed  to  exactly  the  same  amount  of  light,  yet  the  coty- 
ledons in  the  one  pot  were  rising,  whilst  those  in  the  other 
pot  were  at  the  same  time  sinking.  This  fact  illustrates  in  a 
striking  manner  that  their  movements  are  not  governed  by  the 
actual  amount,  but  by  a  change  in  the  intensity  or  degree  of 
the  light.  A  similar  experiment  was  tried  with  two  sets  of  seed- 
lings, both  exposed  to  a  dull  light,  but  different  in  degree,  and 
the  result  was  the  same.  The  movements  of  the  cotyledons  of  this 
Cassia  are,  however,  determined  (as  in  many  other  cases)  largely 
by  habit  or  inheritance,  independently  of  light;  for  seedlings 
which  had  been  moderately  illuminated  during  the  day,  were 
kept  all  night  and  on  the  following  morning  in  complete  dark- 
ness; yet  the  cotyledons  were  partially  open  in  the  morning 
and  remained  open  in  the  dark  for  about  6  h.  The  cotyledons 
in  another  pot,  similarly  treated  on  another  occasion,  were  open 
at  7  A.M.  and  remained  open  in  the  dark  for  4  h.  30  m,,  after 
which  time  they  began  to  close.  Yet  these  same  seedlings,  when 
brought  in  the  middle  of  the  day  from  a  moderately  bright 
into  only  a  moderately  dull  light  raised,  as  we  have  seen,  their 
cotyledons  high  above  the  horizon. 

Sensitiveness  of  Cotyledons  to  contact. — This  subject  does  not 
possess  much  interest,  as  it  is  not  known  that  sensitiveness  of  this 
kind  is  of  any  service  to  seedling  plants.  We  have  observed  cases 
in  only  four  genera,  though  we  have  vainly  observed  the  coty- 
ledons of  many  others.  The  genus  Cassia  seems  to  be  pre-eminent 
in  this  respect :  thus,  the  cotyledons  of  C.  tora,  when  extended 
horizontally,  were  both  lightly  tapped  with  a  very  thin  twig  for 
3  m.,  and  in  the  course  of  a  few  minutes  they  formed  together 
an  angle  of  90°,  so  that  each  had  risen  45°.  A  single  cotyledon 
of  another  seedling  was  tapped  in  a  like  manner  for  1  m.,  and  it 
rose  27°  in  9  m. ;  and  after  eight  additional  minutes  it  had  risen 
10°  more ;  the  opposite  cotyledon,  which  was  not  tapped,  hardly 
moved  at  all.  The  cotyledons  in  all  these  cases  became  hori- 
zontal again  in  less  than  half  an  hour.  The  pulvinus  is  the  most 
sensitive  part,  for  on  slightly  pricking  three  cotyledons  with  a 


126  COTYLEDONS   SENSITIVE  CHAP.  IL 

pin  in  this  part,  they  rose  up  vertically ;  but  the  blade  was  found 
also  to  be  sensitive,  care  having  been  taken  that  the  pulvinus 
was  not  touched.  Drops  of  water  placed  quietly  on  these  coty- 
ledons produced  no  effect,  but  an  extremely  fine  stream  of  water, 
ejected  from  a  syringe,  caused  them  to  move  upwards.  When 
a  pot  of  seedlings  was  rapidly  hit  with  a  stick  and  thus  jarred, 
the  cotyledons  rose  slightly.  When  a  minute  drop  of  nitric 
acid  was  placed  on  both  pulvini  of  a  seedling,  the  cotyledons 
rose  so  quickly  that  they  could  easily  be  seen  to  move,  and 
almost  immediately  afterwards  they  began  to  fall;  but  the 
pulvini  had  been  killed  and  became  brown. 

The  cotyledons  of  an  unnamed  species  of  Cassia  (a  large  tree 
from  S.  Brazil)  rose  31°  in  the  course  of  26  m.  after  the  pulvini 
and  the  blades  had  both  been  rubbed  during  1  m.  with  a  twig ; 
but  when  the  blade  alone  was  similarly  rubbed  the  cotyledons 
rose  only  8°.  The  remarkably  long  and  narrow  cotyledons,  of  a 
third  unnamed  species  from  S.  Brazil,  did  not  move  when  their 
blades  were  rubbed  on  six  occasions  with  a  pointed  stick  for 
30  s.  or  for  1  m. ;  but  when  the  pulvinus  was  rubbed  and  slightly 
pricked  with  a  pin,  the  cotyledons  rose  in  the  course  of  a  few 
minutes  through  an  angle  of  60°.  Several  cotyledons  of 
C.  neghcta  (likewise  from  S.  Brazil)  rose  in  from  5  m.  to  15  m.  to 
various  angles  between  16°  and  34°,  after  being  rubbed  during 
1  m.  with  a  twig.  Their  sensitiveness  is  retained  to  a  somewhat 
advanced  age,  for  the  cotyledons  of  a  little  plant  of  O.  neglecta, 
34  days  old  and  bearing  three  true  leaves,  rose  when  lightly 
pinched  between  the  finger  and  thumb.  Some  seedlings  were 
exposed  for  30  m.  to  a  wind  (temp.  50°  F.)  sufficiently  strong  to 
keep  the  cotyledons  vibrating,  but  this  to  our  surprise  did  not 
cause  any  movement.  The  cotyledons  of  four  seedlings  of  the 
Indian  C.  ylauca  were  either  rubbed  with  a  thin  twig  for  2  m.  or 
were  lightly  pinched:  one  rose  34°;  a  second  only  6°;  a  third 
13°;  and  a  fourth  17°.  A  cotyledon  of  C.  florida  similarly 
treated  rose  9° ;  one  of  C.  corymbosa  rose  7J°,  and  one  of  the 
very  distinct  C.  mimosoides  only  6°.  Those  of  C.  pubescens  did 
not  appear  to  be  in  the  least  sensitive ;  nor  were  those  of  C. 
nt.dosa,  but  these  latter  are  rather  thick  and  fleshy,  and  do  not 
rise  at  night  or  go  to  sleep. 

Smithia  sensitiva.—This  plant  belongs  to  a  distinct  sub-order  of 
the  Leguminosse  from  Cassia.  Both  cotyledons  of  an  oldish 
seedling,  with  the  first  true  leaf  partially  unfolded,  were  rubbed 
for  1  m.  with  a  fine  twig,  and  in  5  m.  each  rose  32°;  they 


CHAP.  II.  TO  CONTACT.  127 

remained  in  tins  position  for  15  m.,  but  when  looked  at  again 
40-m.  after  the  rubbing,  each  had  fallen  14°.  Both  cotyledons  of 
another  and  younger  seedling  were  lightly  rubbed  in  the  same 
manner  for  1  m.,  and  after  an  interval  of  32  m.  each  had  risen 
30°.  They  were  hardly  at  all  sensitive  to  a  fine  jet  of  water. 
The  cotyledons  of  S.  Pfundii,  an  African  water  plant,  are  thick 
and  fleshy ;  they  are  not  sensitive  and  do  not  go  to  sleep. 

Mimosa  pudica  and  albida. — The  blades  of  several  cotyledons 
of  both  these  plants  were  rubbed  or  slightly  scratched  with  a 
needle  during  1m.  or  2  m. ;  but  they  did  not  move  in  the  least. 
When,  however,  the  pulvini  of  six  cotyledons  of  M.  pudica  were 
thus  scratched,  two  of  them  were  slightly  raised.  In  these  two 
cases  perhaps  the  pulvinus  was  accidentally  pricked,  for  on 
pricking  the  pulvinus  of  another  cotyledon  it  rose  a  little.  It 
thus  appears  that  the  cotyledons  of  Mimosa  are  less  sensitive 
than  those  of  the  previously  mentioned  plants.* 

Oxulis  sensitiva. — The  blades  and  pulvini  of  two  cotyledons, 
standing  horizontally,  were  rubbed  or  rather  tickled  for  30s. 
with  a  fine  split  bristle,  and  in  10m.  each  had  risen  48°; 
when  looked  at  again  in  35  m.  after  being  rubbed  they  had 
risen  4°  more ;  after  30  additional  minutes  they  were  again  hori- 
zontal. On  hitting  a  pot  rapidly  with  a  stick  for  1  m.,  the  coty- 
ledons of  two  seedlings  were  considerably  raised  in  the  course 
of  11  m.  A  pot  was  carried  a  little  distance  on  a  tray  and  thus 
jolted ;  and  the  cotyledons  of  four  seedlings  were  all  raised  in 
10  m. ;  after  17  m.  one  had  risen  56°,  a  second  45°,  a  third  almost 
90°,  and  a  fourth  90°.  After  an  additional  interval  of  40  m.  three 
of  them  had  re-expanded  to  a  considerable  extent.  These  obser- 
vations were  made  before  we  were  aware  at  what  an  extraordi- 
narily rapid  rate  the  cotyledons  circumnutate,  and  are  therefore 
liable  to  error.  Nevertheless  it  is  extremely  improbable  that  the 
cotyledons  in  the  eight  cases  given,  should  all  have  been  rising 
at  the  time  when  they  were  irritated.  The  cotyledons  of  Oxalis 
Valdiviana  and  rosea  were  rubbed  and  did  not  exhibit  any 
sensitiveness. 

Finally,  there  seems  to  exist  some  relation  between 


*  The   sole   notice  which  we  p.  865),   "  les  cotyledons  du  M 

have  met  with  on  the  sensitive-  pudica  tendent  a  se  raprocher  par 

ness  of  cotyledons,  relates  to  Mi-  leurs  faces  supe'rieures  lorsqu'on 

mosa ;  for  Au°r.  P.  De  Candolle  les  irrite." 
says  ('Phys.  Veg.,'  1832,  torn,  ii 


128  SENSITIVENESS   OF  COTYLEDONS.        CHAP  II. 

the  habit  of  cotyledons  rising  vertically  at  night  or 
going  to  sleep,  and  their  sensitiveness,  especially  that 
of  their  pulvini,  to  a  touch ;  for  all  the  above-named 
plants  sleep  at  night.  On  the  other  hand,  there  are 
many  plants  the  cotyledons  of  which  sleep,  and  are 
not  in  the  least  sensitive.  As  the  cotyledons  of 
several  species  of  Cassia  are  easily  affected  both  by 
slightly  diminished  light  and  by  contact,  we  thought 
that  these  two  kinds  of  sensitiveness  might  be  con- 
nected ;  but  this  is  not  necessarily  the  case,  for  the 
cotyledons  of  Oxalis  sensitiva  did  not  rise  when  kept 
on  one  occasion  for  1J  h.,  and  on  a  second  occasion 
for  nearly  4  h.,  in  a  dark  closet.  Some  other  coty- 
ledons, as  those  of  Githago  segetum,  are  much  affected 
by  a  feeble  light,  but  do  not  move  when  scratched  by 
a  needle.  That  with  the  same  plant  there  is  some 
relation  between  the  sensitiveness  of  its  cotyledons 
and  leaves  seems  highly  probable,  for  the  above  de- 
scribed Smithia  and  Oxalis  have  been  called  sensitiva, 
owing  to  their  leaves  being  sensitive ;  and  though  the 
leaves  of  the  several  species  of  Cassia  are  not  sensitive 
to  a  touch,  yet  if  a  branch  be  shaken  or  syringed 
with  water,  they  partially  assume  their  nocturnal  de- 
pendent position.  But  the  relation  between  the  sen- 
sitiveness to  contact  of  the  cotyledons  and  of  the 
leaves  of  the  same  plant  is  not  very  close,  as  may  be 
inferred  from  the  cotyledons  of  Mimosa  pudica  being 
only  slightly  sensitive,  whilst  the  leaves  are  well 
known  to  be  so  in  the  highest  degree.  Again,  the 
leaves  of  Neptunia  oleracea  are  very  sensitive  to  a 
touch,  whilst  the  cotyledons  do  not  appear  to  be  so  in 
any  degree. 


CHAP.  III.          SENSITIVENESS   OF   RADICLES.  129 


CHAPTER  III. 

SENSITIVENESS  OF  THE  APEX  or  THE  RADICLE  TO  CONTACT  AND  TO 
OTHER  IRRITANTS. 

Manner  in  which  radicles  bend  when  they  encounter  an  obstacle  in 
the  soil — Vicia  faba,  tips  of  radicles  highly  sensitive  to  contact 
and  other  irritants — Effects  of  too  high  a  temperature — Power  of 
discriminating  between  objects  attached  on  opposite  sides — Tips  of 
secondary  radicles  sensitive — Pisum,  tips  of  radicles  sensitive — 
Effects  of  such  sensitiveness  in  overcoming  geotropism— -Secondary 
radicles — Phaseolus,  tips  of  radicles  hardly  sensitive  to  contact 
but  highly  sensitive  to  caustic  and  to  the  removal  of  a  slice — Tro- 
pscolum — Gossypiuin— Cucurbita — Raphanus— JSsculus,  tip  not 
sensitive  to  slight  contact,  highly  sensitive  to  caustic — Quercus, 
tip  highly  sensitive  to  contact — Power  of  discrimination — Zea 
tip  highly  sensitive,  secondary  radicles— Sensitiveness  of  radicles 
to  moist  air — Summary  of  chapter. 

IN  order  to  see  how  the  radicles  of  seedlings  would 
pass  over  stones,  roots,  and  other  obstacles,  which  they 
must  incessantly  encounter  in  the  soil,  germinating 
beans  (Vicia  faba)  were  so  placed  that  the  tips  of  the 
radicles  came  into  contact,  almost  rectangularly  or 
at  a  high  angle,  with  "underlying  plates  of  glass.  In 
other  cases  the  beans  were  turned  about  whilst  their 
radicles  were  growing,  so  that  they  descended  nearly 
vertically  on  their  own  smooth,  almost  flat,  broad  upper 
surfaces.  The  delicate  root-cap,  when  it  first  touched 
any  directly  opposing  surface,  was  a  little  flattened 
transversely ;  the  flattening  soon  became  oblique,  and 
in  a  few  hours  quite  disappeared,  the  apex  now  point- 
ing at  right  angles,  or  at  nearly  right  angles,  to  its 
former  course.  The  radicle  then  seemed  to  glide  in 
its  new  direction  over  the  surface  which  had  opposed 


130  SENSITIVENESS  OF  RADICLES.       CHAP.  III. 

it,  pressing  on  it  with  very  little  force.  How  far  such 
abrupt  changes  in  its  former  course  are  aided  by  the 
circumnutation  of  the  tip  must  be  left  doubtful.  Thin 
slips  of  wood  were  cemented  on  more  or  less  steeply 
inclined  glass-plates,  at  right  angles  to  the  radicles 
which  were  gliding  down  them.  Straight  lines  had 
been  painted  along  the  growing  terminal  part  of  some 
of  these  radicles,  before  they  met  the  opposing  slip 
of  wood ;  and  the  lines  became  sensibly  curved  in  2  h. 
after  the  apex  had  come  into  contact  with  the  slips. 
In  one  case  of  a  radicle,  which  was  growing  rather 
slowly,  the  root-cap,  after  encountering  a  rough  slip 
of  wood  at  right  angles,  was  at  first  slightly  flat- 
tened transversely :  after  an  interval  of  2  h.  30  m. 
the  flattening  became  oblique ;  and  after  an  addi- 
tional 3  hours  the  flattening  had  wholly  disappeared, 
and  the  apex  now  pointed  at  right  angles  to  its  former 
course.  It  then  continued  to  grow  in  its  new  direc- 
tion alongside  the  slip  of  wood,  until  it  came  to  the 
end  of  it,  round  which  it  bent  rectangularly.  Soon 
afterwards  when  coming  to  the  edge  of  the  plate  of 
glass,  it  was  again  bent  at  a  large  angle,  and  de- 
scended perpendicularly  into  the  damp  sand. 

When,  as  in  the  above  cases,  radicles  encountered 
an  obstacle  at  right  angles  to  their  course,  the  terminal 
growing  part  became  curved  for  a  length  of  between 
•3  and  *4  of  an  inch  (8-10  mm.),  measured  from  the 
apex.  This  was  well  shown  by  the  black  lines  which 
had  been  previously  painted  on  them.  The  first  and 
most  obvious  explanation  of  the  curvature  is,  that  it 
results  merely  from  the  mechanical  resistance  to  the 
growth  of  the  radicle  in  its  original  direction.  Never- 
theless, this  explanation  did  not  seem  to  us  satisfactory. 
The  radicles  did  not  present  the  appearance  of  having 
been  subjected  to  a  sufficient  pressure  to  account  for 


CHAP.  III.          SENSITIVENESS  OF  RADICLES.  131 

their  curvature ;  and  Sachs  has  shown  *  that  the 
growing  part  is  more  rigid  than  the  part  immediately 
above  which  has  ceased  to  grow,  so  that  the  latter 
might  have  been  expected  to  yield  and  become  curved 
as  soon  as  the  apex  encountered  an  unyielding  object ; 
whereas  it  was  the  stiff  growing  part  which  became 
curved.  Moreover,  an  object  which  yields  with  the 
greatest  ease  will  deflect  a  radicle  :  thus,  as  we  have 
seen,  when  the  apex  of  the  radicle  of  the  bean 
encountered  the  polished  surface  of  extremely  thin 
tin-foil  laid  on  soft  sand,  no  impression  was  left  on  it 
yet  the  radicle  became  deflected  at  right  angles.  A 
second  explanation  occurred  to  us,  namely,  that  even 
the  gentlest  pressure  might  check  the  growth  of  the 
apex,  and  in  this  case  growth  could  continue  only  on 
one  side,  and  thus  the  radicle  would  assume  a  rectan- 
gular form  ;  but  this  view  leaves  wholly  unexplained 
the  curvature  of  the  upper  part,  extending  for  a  length 
of  8-10  mm. 

We  were  therefore  led  to  suspect  that  the  apex 
was  sensitive  to  contact,  and  that  an  effect  was  trans- 
mitted from  it  to  the  upper  part  of  the  radicle,  which 
was  thus  excited  to  bend  away  from  the  touching  object. 
As  a  little  loop  of  fine  thread  hung  on  a  tendril  or 
on  the  petiole  of  a  leaf-climbing  plant,  causes  it  to 
bend,  we  thought  that  any  small  hard  object  affixed 
to  the  tip  of  a  radicle,  freely  suspended  and  growing 
in  damp  air,  might  cause  it  to  bend,  if  it  were  sensitive, 
and  yet  would  not  offer  any  mechanical  resistance  to 
its  growth.  Full  details  will  be  given  of  the  experi- 
ments which  were  tried,  as  the  result  proved  remark- 
able. The  fact  of  the  apex  of  a  radicle  being  sensitive 
to  contact  has  never  been  observed,  though,  as  we  shall 


*  « Arbeitcn  Bot.  Inst.  WUrzburg,'  Heft  iii.  1873,  p.  398. 


132  SENSITIVENESS   OF   THE  APEX        CHAP.  III. 

hereafter  see,  Sachs  discovered  that  the  radicle  a  little 
above  the  apex  is  sensitive,  and  bends  like  a  tendril 
towards  the  touching  object.  But  when  one  side  of  the 
apex  is  pressed  by  any  object,  the  growing  part  bends 
away  from  the  object;  and  this  seems  a  beautiful 
adaptation  for  avoiding  obstacles  in  the  soil,  and,  as 
we  shall  see,  for  following  the  lines  of  least  resistance. 
Many  organs,  when  touched,  bend  in  one  fixed  direc- 
tion, such  as  the  stamens  of  Berberis,  the  lobes  of 
Dionaea,  &c. ;  and  many  organs,  such  as  tendrils,  whe- 
ther modified  leaves  or  flower-peduncles,  and  some  few 
stems,  bend  towards  a  touching  object ;  but  no  case, 
we  believe,  is  known  of  an  organ  bending  away  from 
a  touching  object. 

Sensitiveness  of  the  Apex  of  the  Radicle  of  Vicia  faba. 
— Common  beans,  after  being  soaked  in  water  for  24  h., 
were  pinned  with  the  hilum  downwards  (in  the  manner 
followed  by  Sachs),  inside  the  cork  lids  of  glass-vessels, 
which  were  half  filled  with  water;  the  sides  and  the 
cork  were  well  moistened,  and  light  was  excluded. 
As  soon  as  the  beans  had  protruded  radicles,  some  to  a 
length  of  less  than  a  tenth  of  an  inch,  and  others  to 
a  length  of  several  tenths,  little  squares  or  oblongs  of 
card  were  affixed  to  the  short  sloping  sides  of  their 
conical  tips.  The  squares  therefore  adhered  obliquely 
with  reference  to  the  longitudinal  axis  of  the  radicle  ; 
and  this  is  a  very  necessary  precaution,  for  if  the  bits 
of  card  accidentally  became  displaced,  or  were  drawn 
by  the  viscid  matter  employed,  so  as  to  adhere  parallel 
to  the  side  of  the  radicle,  although  only  a  little  way 
above  the  conical  apex,  the  radicle  did  not  bend  in 
the  peculiar  manner  which  we  are  here  considering. 
Squares  of  about  the  ^th  of  an  inch  (i.e.  about  1 J  mm.), 
or  oblong  bits  of  nearly  the  same  size,  were  found  to 


CHAP.  III.       OF  THE  EADICLE   OF  THE  BEAN.  133 

be  the  most  convenient  and  effective.  We  employed 
at  first  ordinary  thin  card,  such  as  visiting  cards,  or 
bits  of  very  thin  glass,  and  various  other  objects ;  but 
afterwards  sand-paper  was  chiefly  employed,  for  it  was 
almost  as  stiff  as  thin  card,  and  the  roughened  surface 
favoured  its  adhesion.  At  first  we  generally  used  very 
thick  gum-water;  and  this  of  course,  under  the  cir- 
cumstances, never  dried  in  the  least ;  on  the  contrary, 
it  sometimes  seemed  to  absorb  vapour,  so  that  the  bits 
of  card  became  separated  by  a  layer  of  fluid  from  the 
tip.  When  there  was  no  such  absorption  and  the  card 
was  not  displaced,  it  acted  well  and  caused  the  radicle 
to  bend  to  the  opposite  side.  I  should  state  that 
thick  gum-water  by  itself  induces  no  action.  In  most 
cases  the  bits  of  card  were  touched  with  an  extremely 
small  quantity  of  a  solution  of  shellac  in  spirits  of 
wine,  which  had  been  left  to  evaporate  until  it  was 
thick ;  it  then  set  hard  in  a  few  seconds,  and  fixed  the 
bits  of  card  well.  When  small  drops  of  the  shellac 
were  placed  on  the  tips  without  any  card,  they  set  into 
hard  little  beads,  and  these  acted  like  any  other  hard 
object,  causing  the  radicles  to  bend  to  the  opposite 
side.  Even  extremely  minute  beads  of  the  shellac 
occasionally  acted  in  a  slight  degree,  as  will  hereafter 
be  described.  But  that  it  was  the  cards  which  chiefly 
acted  in  our  many  trials,  was  proved  by  coating  one 
side  of  the  tip  with  a  little  bit  of  goldbeaters'  skin 
(which  by  itself  hardly  acts),  and  then  fixing  a  bit  of 
card'  to  the  skin  with  shellac  which  never  came  into 
contact  with  the  radicle :  nevertheless  the  radicle  bent 
away  from  the  attached  card  in  the  ordinary  manner. 

Some  preliminary  trials  were  made,  presently  to 
be  described,  by  which  the  proper  temperature  was 
determined,  and  then  the  following  experiments  were 
made.  It  should  be  premised  that  the  beans  were 


134 


SENSITIVENESS   OF  THE  APEX        CHAP.  III. 


always  fixed  to  the  cork-lids,  for  the  convenience  of 
manipulation,  with  the  edge  from  which  the  radicle 
and  plumule  protrudes,  outwards ;  and  it  must  be 
remembered  that  owing  to  what  we  have  called  Sachs' 
curvature,  the  radicles,  instead  of  growing  perpendi- 
cularly downwards,  often  bend  somewhat,  even  as  much 


A. 


c. 


Vicia  fdba :  A,  radicle  beginning  to  bend  from  the  attached  little  square 
of  card ;  B,  bent  at  a  rectangle ;  C,  bent  into  a  circle  or  loop,  with  the 
tip  beginning  to  bend  downwards  through  the  action  of  geotropi.sm. 

as  about  45°  inwards,  or  under  the  suspended  bean. 
Therefore  when  a  square  of  card  was  fixed  to  the  apex 
in  front,  the  bowing  induced  by  it  coincided  with  Sachs' 
curvature,  and  could  be  distinguished  from  it  only  by 
being  more  strongly  pronounced  or  by  occurring  more 
quickly.  To  avoid  this  source  of  doubt,  the  squares 


CHAP.  III.       OF  THE  RADICLE   OF  THE  BEAN.  135 

were  fixed  either  behind,  causing  a  curvature  in  direct 
opposition  to  that  of  Sachs',  or  more  commonly  to  the 
right  or  left  sides.  For  the  sake  of  brevity,  we  will 
speak  of  the  bits  of  card,  &c.,  as  fixed  in  front,  or 
behind,  or  laterally.  As  the  chief  curvature  of  the 
radicle  is  at  a  little  distance  from  the  apex,  and  as 
the  extreme  terminal  and  basal  portions  are  nearly 
straight,  it  is  possible  to  estimate  in  a  rough  manner 
the  amount  of  curvature  by  an  angle  ; .  and  when  it  is 
said  that  the  radicle  became  deflected  at  any  angle 
from  the  perpendicular,  this  implies  that  the  apex  was 
turned  upwards  by  so  many  degrees  from  the  down- 
ward direction  which  it  would  naturally  have  followed, 
and  to  the  side  opposite  to  that  to  which  the  card  was 
affixed.  That  the  reader  may  have  a  clear  idea  of  the 
kind  of  movement  excited  by  the  bits  of  attached 
card,  we  append  here  accurate  sketches  of  three  ger- 
minating beans  thus  treated,  and  selected  out  of 
several  specimens  to  show  the  gradations  in  the 
degrees  of  curvature.  We  will  now  give  in  detail  a 
series  of  experiments,  and  afterwards  a  summary  of 
the  results. 

* 

In  the  first  12  trials,  little  squares  or  oblongs  of  sanded  card, 
1'8  mm.  in  length,  and  T5  or  only  0*9  mm.  in  breadth  (i.e.  '071 
ot  an  inch  in  length  and  *059  or  '035  of  an  inch  in  breadth)  were 
fixed  with  shellac  to  the  tips  of  the  radicles.  In  the  subsequent 
trials  the  little  squares  were  only  occasionally  measured,  but 
were  of  about  the  same  size. 

(1.)  A  young  radicle.  4  mm.  in  length,  had  a  card  fixed  be- 
hind: after  9  h.  deflected  in  the  plane  in  which  the  bean  is 
flattened,  50°  from  the  perpendicular  and  from  the  card,  and  in 
opposition  to  Sachs'  curvature  :  no  change  next  morning,  23  h. 
from  the  time  of  attachment. 

(2.)  Eadicle  5'5  mm.  in  length,  card  fixed  behind :  after  9  h. 
deflected  in  the  plane  of  the  bean  20°  from  the  perpendicular 
and  from  the  card,  and  in  opposition  to  Sachs'  curvature :  after 
23  h.  no  change. 
7 


136  SENSITIVENESS  OF   THE   APEX        CHAP.  III. 

(3.)  Radicle  11  mm.  in  length,  card  fixed  behind:  after  9 h. 
deflected  in  the  plane  of  the  bean  40°  from  the  perpendicular 
and  from  the  card,  and  in  opposition  to  Sachs'  curvature.  The 
tip  of  the  radicle  more  curved  than  the  upper  part,  but  in  the 
same  plane.  After  23  h.  the  extreme  tip  was  slightly  bent  to- 
wards the  card;  the  general  curvature  of  the  radicle  remaining 
the  same. 

(4.)  Radicle  9  mm.  long,  card  fixed  behind  and  a  little 
laterally:  after  9h.  deflected  in  the  plane  of  the  bean  only 
about  7°  or  8°  from  the  perpendicular  and  from  the  card-,  in 
opposition  to  Sachs*  curvature.  There  was  in  addition  a  slight 
lateral  curvature  directed  partly  from  the  card.  After  '23  h.  no 
change. 

(5.)  Radicle  8  mm.  long,  card  affixed  almost  laterally :  after 
9  h.  deflected  30°  from  the  perpendicular,  in  the  plane  of  the 
bean  and  in  opposition  to  Sachs'  curvature ;  also  deflected  in  a 
plane  at  right  angles  to  the  above  one,  20°  from  the  perpen- 
dicular: after  23  h.  no  change. 

(6.)  Radicle  9  mm.  long,  card  affixed  in  front :  after  9  h.  de- 
flected in  the  plane  of  the  bean  about  40°  from  the  vertical, 
away  from  the  card  and  in  the  direction  of  Sachs'  curvature. 
Here  therefore  we  have  no  evidence  of  the  card  being  the 
cause  of  the  deflection,  except  that  a  radicle  never  moves 
spontaneously,  as  far  as  we  have  seen,  as  much  as  40°  in  the 
course  of  9  h.  After  23  h.  no  change. 

(7.)  Radicle  7  mm.  long,  card  affixed  to  the  back :  after  9  h. 
the  terminal  part  of  the  radicle  deflected  in  the  plane  of  the 
bean  20°  from  the  vertical,  away  from  the  card  and  in  opposition 
to  Sachs'  curvature.  After  22  h.  30  m.  this  part  of  the  radicle 
had  become  straight. 

(8.)  Radicle  12  mm.  long,  card  affixed  almost  laterally :  after 
9  h.  deflected  laterally  in  a  plane  at  right  angles  to  that  of  the 
bean  between  40°  and  50°  from  the  vertical  and  from  the  card. 
In  the  plane  of  the  bean  itself  the  deflection  amounted  to  8°  or 
9°  from  the  vertical  and  from  the  card,  in  opposition  to  Sachs' 
curvature.  After  22  h.  30  in.  the  extreme  tip  had  become 
slightly  curved  towards  the  card. 

(9.)  Card  fixed  laterally:  after  11  h.  30m  no  effect,  the 
radicle  being  still  almost  vertical. 

(10.)  Card  fixed  almost  laterally :  after  11  h.  30  m.  deflected 
90°  from  the  vertical  and  from  the  card,  in  a  plane  inter- 
mediate between  that  of  the  bean  itself  and  one  at  right 


CHAP.  III.       OF  THE  KADICLE   OF   THE   BEAN.  137 

angles  to  it.  Radicle  consequently  partially  deflected  from 
Sachs'  curvature. 

(11.)  Tip  of  radicle  protected  with  goldbeaters'  skin,  with  a 
square  of  card  of  the  usual  dimensions  affixed  with  shellac : 
after  11  h.  greatly  deflected  in  the  plane  of  the  bean,  in  the 
direction  of  Sachs'  curvature,  but  to  a  much  greater  degree  and 
in  less  time  than  ever  occurs  spontaneously. 

(12.)  Tip  of  radicle  protected  as  in  last  case :  after  11  h.  no 
effect,  but  after  24  h.  40m.  radicle  clearly  deflected  from  the 
card.  This  slow  action  was  probably  due  to  a  portion  of  the 
goldbeaters'  skin  having  curled  round  and  lightly  touched  the 
opposite  side  of  the  tip  and  thus  irritated  it. 

(13.)  A  radicle  of  considerable  length  had  a  small  square  of 
card  fixed  with  shellac  to  its  apex  laterally :  after  only  7  h.  15  m. 
a  length  of  '4  of  an  inch  from  the  ,apex,  measured  along  the 
middle,  was  considerably  curved  from  the  side  bearing  the  card. 

(14.)  Case  like  the  last  in  all  respects,  except  that  a  length  of 
only  '25  of  an  inch  of  the  radicle  was  thus  deflected. 

(15.)  A  small  square  of  card  fixed  with  shellac  to  the  apex  of 
a  young  radicle ;  after  9  h.  15  m.  deflected  through  90°  from  the 
perpendicular  and  from  the  card.  After  24  h.  deflection  much 
decreased,  and  after  an  additional  day,  reduced  to  23°  from  the 
perpendicular. 

(16.)  Square  of  card  fixed  with  shellac  behind  the  apex  of  a 
radicle,  which  from  its  position  having  been  changed  during 
growth  had  become  very  crooked;  but  the  terminal  portion 
was  straight,  and  this  became  deflected  to  about  45°  from 
the  perpendicular  and  from  the  card,  in  opposition  to  Sachs' 
curvature. 

(17.)  Square  of  card  affixed  with  shellac :  after  8  h.  radicle 
curved  at  right  angles  from  the  perpendicular  and  from  the 
card.  After  15  additional  hours  curvature  much  decreased. 

(18.)  Square  of  card  affixed  with  shellac :  after  8  h.  no  effect ; 
after  23  h.  3  m.  from  time  of  affixing,  radicle  much  curved  from 
the  square. 

(19.)  Square  of  card  affixed  with  shellac :  after  24  h.  no  effect, 
but  the  radicle  had  not  grown  well  and  seemed  sickly. 

(20.)  Square  of  card  affixed  with  shellac :  after  24  h.  no  effect. 

(21,  22.)  Squares  of  card  affixed  with  shellac:  after  24 h. 
radicles  of  both  curved  at  about  45°  from  the  perpendicular  and 
from  the  cards. 

(23.)  Square  of  card  fixed  with  shellac  to  young  radicle :  after 


138  SENSITIVENESS   OF   THE  APEX          CHAP.  III. 

9  h.  very  slightly  curved  from  the  card ;  after  24  h.  tip  curved 
towards  card.  Kefixed  new  square  laterally,  atter  9  h.  distinctly 
curved  from  the  card,  and  after  24  h.  curved  at  right  angles  from 
the  perpendicular  and  from  the  card. 

(24.)  A  rather  large  oblong  piece  of  card  fixed  with  shellac  to 
apex :  after  24  h.  no  effect,  but  the  card  was  found  not  to  be 
touching  the  apex.  A  small  square  was  now  refixed  with 
shellac ;  atter  16  h.  slight  deflection  from  the  perpendicular 
and  from  the  card.  After  an  additional  day  the  radicle  became 
almost  straight. 

(25.)  Square  of  card  fixed  laterally  to  apex  of  young  radicle ; 
after  9  h.  deflection  from  the  perpendicular  considerable ;  after 
24  h.  deflection  reduced.  Kefixed  a  fresh  square  with  shellac : 
after  24  h.  deflection  about  40°  from  the  perpendicular  and  from 
the  card. 

(26.)  A  very  small  square  of  card  fixed  with  shellac  to  apex  of 
young  radicle  :  after  9  h.  the  deflection  from  the  perpendicular 
and  from  the  card  amounted  to  nearly  a  right  angle;  after  24 h. 
deflection  much  reduced ;  after  an  additional  24  h.  radicle  almost 
straight. 

(27.)  Square  of  card  fixed  with  shellac  to  apex  of  young 
radicle  :  after  9  h.  deflection  from  the  card  and  from  the  perpen- 
dicular a  right  angle ;  next  morning  quite  straight.  Refixed 
a  square  laterally  with  shellac ;  after  9  h.  a  little  deflection, 
which  after  24  h.  increased  to  nearly  20°  from  the  perpendicular 
and  from  the  card. 

(23.)  Square  of  card  fixed  with  shellac;  after  9  h.  some 
deflection ;  next  morning  the  card  dropped  off;  refixed  it  with 
shellac ;  it  again  became  loose  and  was  refixed ;  and  now  on  the 
third  trial  the  radicle  was  deflected  after  14  h.  at  right  angles 
from  the  card. 

(29.)  A  small  square  of  card  was  first  fixed  with  thick  gum- 
water  to  the  apex.  It  produced  a  slight  effect  but  soon  fell 
off.  A  similar  square  was  now  affixed  laterally  with  shellac : 
after  9  h.  the  radicle  was  deflected  nearly  45°  from  the  perpen- 
dicular and  from  the  card.  After  36  additional  hours  angle  of 
deflection  reduced  to  about  30°. 

(30.)  A  very  small  piece,  less  than  -J^th  of  an  inch  square,  of 
thin  tin-foil  fixed  with  shellac  to  the  apex  of  a  young  radicle ; 
after  24  h.  no  effect.  Tin- foil  removed,  and  a  small  square  of 
sanded  card  fixed  with  shellac ;  after  9  h.  deflection  at  nearly 
right  angles  from  the  perpendicular  and  from  the  card.  Next 


CHAP.  III.       OF  THE  KADICLE   OF   THE  BEAN.  139 

morning  deflection  reduced  to  about  40°  from  the  perpen- 
dicular. 

(31.)  A  splinter  of  thin  glass  gummed  to  apex,  after  9  h.  no 
effect,  but  it  was  then  found  not  to  be  touching  the  apex  of  the 
radicle.  Next  morning  a  square  of  card  was  fixed  with  shellac 
to  it,  and  after  9  h.  radicle  greatly  deflected  from  the  card. 
After  two  additional  days  the  deflection  had  decreased  and  was 
only  35°  from  the  perpendicular. 

(32.)  Small  square  of  sanded  card,  attached  with  thick  gum- 
water  laterally  to  the  apex  of  a  long  straight  radicle :  after  9  h. 
greatly  deflected  from  the  perpendicular  and  from  the  card. 
Curvature  extended  for  a  length  of  '22  of  an  inch  from  the 
apex.  After  3  additional  hours  terminal  portion  deflected  at 
right  angles  from  the  perpendicular.  Next  morning  the  curved 
portion  was  '36  in  length. 

(33.)  Square  of  card  gummed  to  apex  :  after  ]5  h.  deflected  at 
nearly  90°  from  the  perpendicular  and  from  the  card. 

(34.)  Small  oblong  of  sanded  card  gummed  to  apex :  after 
15  h.  deflected  90°  from  the  perpendicular  and  from  the  card : 
in  the  course  of  the  three  following  days  the  terminal  portion 
became  much  contorted  and  ultimately  coiled  into  a  helix. 

(35.)  Square  of  card  gummed  to  apex:  after  9  h.  deflected  from 
card :  after  24  h.  from  time  of  attachment  greatly  deflected 
obliquely  and  partly  in  opposition  to  Sachs'  curvature. 

(36.)  Small  piece  of  card,  rathef  less  than  -J^th  of  an  inch 
square,  gummed  to  apex :  in  9  h.  considerably  deflected  from 
card  and  in  opposition  to  Sachs'  curvature ;  after  24  h.  greatly 
deflected  in  the  same  direction.  After  an  additional  day  the 
extreme  tip  was  curved  towards  the  card. 

(37.)  Square  of  card,  gummed  to  apex  in  front,  caused  after 
8  h.  30  m.  hardly  any  effect ;  refixed  fresh  square  laterally,  after 
15  h.  deflected  almost  90°  from  the  perpendicular  and  from  the 
card.  After  2  additional  days  deflection  much  reduced. 

(38.)  Square  of  card  gummed  to  apex :  after  9  h.  much  deflec- 
tion, which  after  24  h.  from  time  of  fixing  increased  to  nearly 
90°.  After  an  additional  day  terminal  portion  was  curled  into 
a  loop,  and  on  the  following  day  into  a  helix. 

(39.)  Small  oblong  piece  of  card  gummed  to  apex,  nearly  in 
front,  but  a  little  to  one  side ;  in  9  h.  slightly  deflected  in  the 
direction  of  Sachs'  curvature,  but  rather  obliquely,  and  to 
side  opposite  to  card.  Next  day  more  curved  in  the  same 
direction,  and  after  2  additional  days  coiled  into  a  ring. 


140  SENSITIVENESS   OF  THE  APEX        CHAP.  III. 

(40.)  Square  of  card  gumined  to  apex:  after  9  h.  slightly 
curved  from  card ;  next  morning  radicle  straight,  and  apex  had 
grown  beyond  the  card.  Eefixed  another  square  laterally  with 
shellac ;  in  9  h.  deflected  laterally,  but  also  in  the  direction  of 
Sachs'  curvature.  After  2  additional  days'  curvature  consider- 
ably increased  in  the  same  direction. 

(41.)  Little  square  of  tin-foil  fixed  with  gum  to  one  side  of 
apex  of  a  young  and  short  radicle :  after  15  h.  no  effect,  but 
tin-foil  had  become  displaced.  A  little  square  of  card  was  now 
gummed  to  one  side  of  apex,  which  after  8  h.  40  m.  was  slightly 
deflected ;  in  24  h.  from  the  time  of  attachment  deflected  at  90° 
from  the  perpendicular  and  from  the  card ;  after  9  additional 
hours  became  hooked,  with  the  apex  pointing  to  the  zenith.  In 
3  days  from  the  time  of  attachment  the  terminal  portion  of  the 
radicle  formed  a  ring  or  circle. 

(42.)  A  little  square  of  thick  letter-paper  gummed  to  the 
apex  of  a  radicle,  which  after  9  h.  was  deflected  from  it.  In 
24  h.  from  time  when  the  paper  was  affixed  the  deflection  much 
increased,  and  after  2  additional  days  it  amounted  to  50°  from 
the  perpendicular  and  from  the  paper. 

(43.)  A  narrow  chip  of  a  quill  was  fixed  with  shellac  to  the 
apex  of  a  radicle.  After  9  h.  no  effect ;  after  24  h.  moderate 
deflection,  but  now  the  quill  had  ceased  to  touch  the  apex. 
Eemoved  quill  and  gummed  a  little  square  of  card  to  apex, 
which  after  8  h.  caused  slight  deflection.  On  the  fourth  day 
from  the  first  attachment  of  any  object,  the  extreme  tip  was 
curved  towards  the  card. 

(44.)  A  rather  long  and  narrow  splinter  of  extremely  thin 
glass,  fixed  with  shellac  to  apex,  it  caused  in  9  h.  slight 
deflection,  which  disappeared  in  24  h. ;  the  splinter  was  then 
found  not  touching  the  apex.  It  was  twice  refixed,  with  nearly 
similar  results,  that  is,  it  caused  slight  deflection,  which  soon 
disappeared.  On  the  fourth  day  from  the  time  of  first  attach- 
ment the  tip  was  bent  towards  the  splinter. 

From  these  experiments  it  is  clear  that  the  apex  of 
the  radicle  of  the  bean  is  sensitive  to  contact,  and 
that  it  causes  the  upper  part  to  bend  away  from  the 
touching  object.  But  before  giving  a  summary  of  the 
results,  it  will  be  convenient  briefly  to  give  a  few  other 
observations.  Bits  of  very  thin  glass  and  little  squares 


CHAP.  IIL       OF  THE  KADICLE   OF    THE  BEAN.  141 

of  common  card  were  affixed  with  thick  gum- water  to 
the  tips  of  the  radicles  of  seven  beans,  as  a  pre- 
liminary trial.  Six  of  these  were  plainly  acted  on, 
and  in  two  cases  the  radicles  became  coiled  up  into 
complete  loops.  One  radicle  was  curved  into  a  semi- 
circle in  so  short  a  period  as  6  h.  10  m.  The 
seventh  radicle  which  was  not  affected  was  apparently 
sickly,  as  it  became  brown  on  the  following  day ;  so 
that  it  formed  no  real  exception.  Some  of  these  trials 
were  made  in  the  early  spring  during  cold  weather  in 
a  sitting-room,  and  others  in  a  greenhouse,  but  the 
temperature  was  not  recorded.  These  six  striking 
cases  almost  convinced  us  that  the  apex  was  sensitive, 
but  of  course  we  determined  to  make  many  more  trials. 
As  we  had  noticed  that  the  radicles  grew  much  more 
quickly  when  subjected  to  considerable  heat,  and  as 
we  imagined  that  heat  would  increase  their  sensitive- 
ness, vessels  with  germinating  beans  suspended  in 
damp  air  were  placed  on  a  chimney-piece,  where  they 
were  subjected  during  the  greater  part  of  the  day  to  a 
temperature  of  between  69°  and  72°  F. ;  some,  how- 
ever, were  placed  in  the  hot-house  where  the  tempera- 
ture was  rather  higher.  Above  two  dozen  beans  were 
thus  tried;  and  when  a  square  of  glass  or  card  did 
not  act,  it  was  removed,  and  a  fresh  one  affixed,  this 
being  often  done  thrice  to  the  same  radicle.  There- 
fore between  five  and  six  dozen  trials  were  altogether 
made.  But  there  was  moderately  distinct  deflection 
from  the  perpendicular  and  from  the  attached  object 
in  only  one  radicle  out  of  this  large  number  of  cases. 
In  five  other  cases  there  was  very  slight  and  doubtful 
deflection.  We  were  astonished  at  this  result,  and 
concluded  that  we  had  made  some  inexplicable  mis- 
take in  the  first  six  experiments.  But  before  finally 
relinquishing  the  subject,  we  resolved  to  make  one 


142  SENSITIVENESS   OF   THE  APEX          CHAP.  III. 

other  trial,  for  it  occurred  to  us  that  sensitiveness  is 
easily  affected  by  external  conditions,  and  that  radicles 
growing  naturally  in  the  earth  in  the  early  spring 
would  not  be  subjected  to  a  temperature  nearly  so 
high  as  70°  F.  We  therefore  allowed  the  radicles 
of  12  beans  to  grow  at  a  temperature  of  between 
55°  and  60°  F.  The  result  was  that  in  every  one  of 
these  cases  (included  in  the  above-described  experi- 
ments) the  radicle  was  deflected  in  the  course  of  a  few 
hours  from  the  attached  object.  All  the  above  re- 
corded successful  trials,  and  some  others  presently  to 
be  given,  were  made  in  a  sitting-room  at  the  tempera- 
tures just  specified.  It  therefore  appears  that  a  tem- 
perature of  about,  or  rather  above,  70°  F.  destroys 
the  sensitiveness  of  the  radicles,  either  directly,  or 
indirectly  through  abnormally  accelerated  growth  ; 
and  this  curious  fact  probably  explains  why  Sachs, 
who  expressly  states  that  his  beans  were  kept  at  a 
high  temperature,  failed  to  detect  the  sensitiveness  of 
the  apex  of  the  radicle. 

But  other  causes  interfere  with  this  sensibility. 
Eighteen  radicles  were  tried  with  little  squares  of 
sanded  card,  some  affixed  with  shellac  and  some  with 
gum- water,  during  the  few  last  days  of  1878,  and  few 
first  days  of  the  next  year.  They  were  kept  in  a  room 
at  the  proper  temperature  during  the  day,  but  were 
probably  too  cold  at  night,  as  there  was  a  hard  frost  at 
the  time.  The  radicles  looked  healthy  but  grew  very 
slowly.  The  result  was  that  only  6  out  of  the  18 
were  deflected  from  the  .attached  cards,  and  this  only 
to  a  slight  degree  and  at  a  very  slow  rate.  These 
radicles  therefore  presented  a  striking  contrast  with 
the  44  above  described.  On  March  6th  and  7th,  when 
the  temperature  of  the  room  varied  between  53°  and 
59°  F.,  eleven  germinating  beans  were  tried  in  the 


CHAP.  III.        OF  THE   KADICLE   OF   THE   BEAN.  143 

same  manner,  and  now  every  one  of  the  radicles 
became  curved  away  from  the  cards,  though  one  was 
only  slightly  deflected.  Some  horticulturists  believe 
that  certain  kinds  of  seeds  will  not  germinate  pro- 
perly in  the  middle  of  the  winter,  although  kept  at  a 
right  temperature.  If  there  really  is  any  proper  period 
for  the  germination  of  the  bean,  the  feeble  degree  of 
sensibility  of  the  above  radicles  may  have  resulted 
from  the  trial  having  been  made  in  the  middle  of  the 
winter,  and  not  simply  from  the  nights  being  too  cold. 
Lastly,  the  radicles  of  four  beans,  which  from  some 
innate  cause  germinated  later  than  all  the  others  of 
the  same  lot,  and  which  grew  slowly  though  appearing 
healthy,  were  similarly  tried,  and  even  after  24  h.  they 
were  hardly  at  all  deflected  from  the  attached  cards. 
We  may  therefore  infer  that  any  cause  which  renders 
the  growth  of  the  radicles  either  slower  or  more  rapid 
than  the  normal  rate,  lessens  or  annuls  the  sensibility 
of  their  tips  to  contact.  It  deserves  particular  atten- 
tion that  when  the  attached  objects  failed  to  act,  there 
was  no  bending  of  any  kind,  excepting  Sachs'  curva- 
ture. The  force  of  our  evidence  would  have  been 
greatly  weakened  if  occasionally,  though  rarely,  the 
radicles  had  become  curved  in  any  direction  inde- 
pendently of  the  attached  objects.  In  the  foregoing 
numbered  paragraphs,  however,  it  may  be  observed 
that  the  extreme  tip  sometimes  becomes,  after  a  con- 
siderable interval  of  time,  abruptly  curved  towards  the 
bit  of  card ;  but  this  is  a  totally  distinct  phenomenon, 
as  will  presently  be  explained. 

Summary  of  the  Results  of  the  foregoing  Experiments 
on  the  Radicles  of  Viciafala. — Altogether  little  squares 
(about  ^th  of  an  inch),  generally  of  sanded  paper 
as  stiff  as  thin  card  (between  '15  and  '20  mm.  in 
thickness),  sometimes  of  ordinary  card,  or  little  frag- 


144  SENSITIVENESS  OF  THE   APEX          CHAP.  III. 

ments  of  very  thin  glass,  &c.,  were*  affixed  at  different 
times  to  one  side  of  the  conical  tips  of  55  radicles. 
The  11  last-mentioned  cases,  but  not  the  preliminary 
ones,  are  .here  included.  The  squares,  &c.,  were  most 
commonly  affixed  with  shellac,  but  in  19  cases  with 
thick  gum-water.  When  the  latter  was  used,  the 
squares  were  sometimes  found,  as  previously  stated, 
to  be  separated  from  the  apex  by  a  layer  of  thick 
fluid,  so  that  there  was  no  contact,  and  conse- 
quently no  bending  of  the  radicle;  and  such  few 
cases  were  not  recorded.  But  in  every  instance  in 
which  shellac  was  employed,  unless  the  square  fell 
off  very  soon,  the  result  was  recorded.  In  several 
instances  when  the  squares  became  displaced,  so  as 
to  stand  parallel  to  the  radicle,  or  were  separated  by 
fluid  from  the  apex,  or  soon  fell  off,  fresh  squares 
were  attached,  and  these  cases  (described  under  the 
numbered  paragraphs)  are  here  included.  Out  of 
55  radicles  experimented  on  under  the  proper  tempe- 
rature, 52  became  bent,  generally  to  a  considerable 
extent  from  the  perpendicular,  and  away  from  the 
side  to  which  the  object  was  attached.  Of  the  three 
failures,  one  can  be  accounted  for,  as  the  radicle 
became  sickly  on  the  following  day;  and  a  second 
was  observed  only  during  11  h.  30  m.  As  in  several 
cases  the  terminal  growing  part  of  the  radicle  continued 
for  some  time  to  bend  from  the  attached  object,  it 
formed  itself  into  a  hook,  with  the  apex  pointing  to 
the  zenith,  or  even  into  a  ring,  and  occasionally  into  a 
spire  or  helix.  It  is  remarkable  that  these  latter  cases 
occurred  more  frequently  when  objects  were  attached 
with  thick  gum-water,  which  never  became  dry,  than 
when  shellac  was  employed.  The  curvature  was  often 
well-marked  in  from  7  h.  to  11  h. ;  and  in  one  instance 
a  semicircle  was  formed  in  6  h.  10  m.  from  the  time 


CHAP.  III.       OF  THE  RADICLE  OF  THE  BEAN.  145 

of  attachment.  Bat  in  order  to  see  the  phenomenon 
as  well  displayed  as  in  the  above  described  cases,  it  is 
indispensable  that  the  bits  of  card,  &c.,  should  be 
made  to  adhere  closely  to  one  side  of  the  conical 
apex;  that  healthy  radicles  should  be  selected  and 
kept  at  not  too  high  or  too  low  a  temperature,  and 
apparently  that  the  trials  should  not  be  made  in  the 
middle  of  the  winter. 

In  ten  instances,  radicles  which  had  curved  away 
from  a  square  of  card  or  other  object  attached  to  their 
tips,  straightened  themselves  to  a  certain  extent,  or 
even  completely,  in  the  course  of  from  one  to  two  days 
from  the  time  of  attachment.  This  was  more  espe- 
cially apt  to  occur  when  the  curvature  was  slight. 
But  in  one  instance  (No.  27)  a  radicle  which  in  9  h. 
had  been  deflected  about  90°  from  the  perpendicular, 
became  quite  straight  in  24  h.  frfjn  the  period  of 
attachment.  With  No.  26,  the  radicle  was  almost 
straight  in  48  h.  We  at  first  attributed  the  straighten- 
ing process  to  the  radicles  becoming  accustomed  to  a 
slight  stimulus,  in  the  same  manner  as  a  tendril  or 
sensitive  petiole  becomes  accustomed  to  a  very  light 
loop  of  thread,  and  unbends  itself  though  tho  loop 
remains  still  suspended ;  but  Sachs  states  *  that  radicles 
of  the  bean  placed  horizontally  in  damp  air  after 
curving  downwards  through  geotropism,  straighten 
themselves  a  little  by  growth  along  their  lower  or 
concave  sides.  Why  this  should  occur  is  not  clear ; 
but  perhaps  it  likewise  occurred  in  the  above  ten 
cases.  There  is  another  occasional  movement  which 
must  not  be  passed  over :  the  tip  of  the  radicle,  for  a 
length  of  from  2  to  3  mm.,  was  found  in  six  instances, 


*  l  Arbeiten  Bot.  Instit.,  Wurzburg,'  Heft  iii.  p.  456. 


146  SENSITIVENESS   OF  THE  APEX        CHAP.  III. 

after  an  interval  of  about  24  or  more  hours,  bent 
towards  the  bit  of  still  attached  card, — that  is,  in  a 
direction  exactly  opposite  to  the  previously  induced 
curvature  of  the  whole  growing  part  for  a  length  of 
from  7  to  8  mm.  This  occurred  chiefly  when  the  first 
curvature  was  small,  and  when  an  object  had  been 
affixed  more  than  once  to  the  apex  of  the  same  radicle. 
The  attachment  of  a  bit  of  card  by  shellac  to  one 
side  of  the  tender  apex  may  sometimes  mechanically 
prevent  its  growth ;  or  the  application  of  thick  gum- 
water  more  than  once  to  the  same  side  may  injure  it; 
and  then  checked  growth  on  this  side  with  continued 
growth  on  the  opposite  and  unaffected  side  would 
account  for  the  reversed  curvature  of  the  apex. 

Various  trials  were  made  for  ascertaining,  as  far 
as  we  could,  the  nature  and  degree  of  irritation  to 
which  the  apex^nust  be  subjected,  in  order  that  the 
terminal  growing  part  should  bend  away,  as  if  to 
avoid  the  cause  of  irritation.  We  have  seen  in  the 
numbered  experiments,  that  a  little  square  of  rather 
thick  letter-paper  gummed  to  the  apex  induced, 
though  slowly,  considerable  deflection.  Judging  from 
several  cases  in  which  various  objects  had  been  affixed 
with  gum,  and  had  soon  become  separated  from  the 
apex  by  a  layer  of  fluid,  as  well  as  from  some  trials 
in  which  drops  of  thick  gum-water  alone  had  been 
applied,  this  fluid  never  causes  bending.  We  have 
also  seen  in  the  numbered  experiments  that  narrow 
splinters  of  quill  and  of  very  thin  glass,  affixed  with 
shellac,  caused  only  a  slight  degree  of  deflection,  and 
this  may  perhaps  have  been  due  to  the  shellac 
itself.  Little  squares  of  goldbeaters'  skin,  which  is 
excessively  -  thin,  were  damped,  and  thus  made  to 
adhere  to  one  side  of  the  tips  of  two  radicles. ;  one  of 
these,  after  24  h.,  produced  no  effect ;  nor  did  the 


CHAP.  III.       OF  THE   RADICLE   OF   THE  BEAN.  147 

other  in  8  h.,  within  which  time  squares  of  card  usually 
act ;  but  after  24  h.  there  was  slight  deflection. 

An  oval  bead,  or  rather  cake,  of  dried  shellac, 
1  •  01  mm.  in  length  and  0  *  63  in  breadth,  caused  a 
radicle  to  become  deflected  at  nearly  right  angles  in 
the  course  of  only  6  h. ;  but  after  23  h.  it  had  nearly 
straightened  itself.  A  very  small  quantity  of  dissolved 
shellac  was  spread  over  a  bit  of  card,  and  the  tips  of 
9  radicles  were  touched  laterally  with  it ;  only  two  of 
them  became  slightly  deflected  to  the  side  opposite 
to  that  bearing  the  speck  of  dried  shellac,  and  they 
afterwards  straightened  themselves.  These  specks 
were  removed,  and  both  together  weighed  less  than 
7(joth  of  a  grain ;  so  that  a  weight  of  rather  less 
than  200^-  °f  a  grain  (0*32  mgs.)  sufficed  to  excite 
movement  in  two  out  of  the  nine  radicles.  Here 
then  we  have  apparently  reached  nearly  the  minimum 
weight  which  will  act. 

A  moderately  thick  bristle  (which  on  measurement 
was  found  rather  flattened,  being  Om33  mm.  in  one 
diameter,  and  0'20  mm.  in  the  other)  was  cut  into 
lengths  of  about  Jo^h  °f  an  incn-  These  after  being 
touched  with  thick  gum-water,  were  placed  on  the  tip 
of  eleven  radicles.  Three  of  them  were  affected ;  one 
being  deflected  in  8  h.  15  m.  to  an  angle  of  about  90° 
from  the  perpendicular ;  a  second  to  the  same  amount 
when  looked  at  after  9  h. ;  but  after  24  h.  from  the 
time  of  first  attachment  the  deflection  had  decreased 
to  only  19° ;  the  third  was  only  slightly  deflected 
after  9  h.,  and  the  bit  of  bristle  was  then  found  not 
touching  the  apex;  it  was  replaced,  and  after  15 
additional  hours  the  deflection  amounted  to  26°  from 
the  perpendicular.  The  remaining  eight  radicles 
were  not  at  all  acted  on  by  the  bits  of  bristle,  so  that 
we  here  appear  to  have  nearly  reached  the  minimum 


148  SENSITIVENESS  OF  THE  APEX        CHAP.  III. 

of  size  of  an  object  which  will  act  on  the  radicle  of 
the  bean.  But  it  is  remarkable  that  when  the  bits  of 
bristle  did  act,  that  they  should  have  acted  so  quickly 
and  efficiently. 

As  the  apex  of  a  radicle  in  penetrating  the  ground 
must  be  pressed  on  all  sides,  we  wished  to  learn 
whether  it  could  distinguish  betwreen  harder  or  more 
resisting,  and  softer  substances.  A  square  of  the  sanded 
paper,  almost  as  stiff  as  card,  and  a  square  of  extremely 
thin  paper  (too  thin  for  writing  on),  of  exactly  the 
same  size  (about  gV^  °f  an  inch),  were  fixed  with 
shellac  on  opposite  sides  of  the  apices  of  12  suspended 
radicles.  The  sanded  card  was  between  0'15  and 
0-20  mm.  (or  between  0-0059  and  0'0079  of  an  inch), 
and  the  thin  paper  only  0  •  045  mm.  (or  0  •  00176  of  an 
inch)  in  thickness.  In  8  out  of  the  12  cases  there 
could  be  no  doubt  that  the  radicle  was  deflected  from 
the  side  to  which  the  card-like  paper  was  attached,  and 
towards  the  opposite  side,  bearing  the  very  thin  paper. 
This  occurred  in  some  instances  in  9  h.,  but  in  others 
not  until  24  h.  had  elapsed.  Moreover,  some  of  the 
four  failures  can  hardly  be  considered  as  really  failures  : 
thus,  in  one  of  them,  in  which  the  radicle  remained 
quite  straight,  the  square  of  thin  paper  was  found, 
when  both  were  removed  from  the  apex,  to  have  been 
so  thickly  coated  with  shellac  that  it  was  almost  as 
stiff  as  the  card :  in  the  second  case,  the  radicle  was 
bent  upwards  into  a  semicircle,  but  the  deflection 
was  not  directly  from  the  side  bearing  the  card,  and 
this  was  explained  by  the  two  squares  having  become 
cemented  laterally  together,  forming  a  sort  of  stiff 
gable,  from  which  the  radicle  was  deflected :  in  the 
third  case,  the  square  of  card  had  been  fixed  by 
mistake  in  front,  and  though  there  was  deflection 
from  it,  this  might  have  been  due  to  Sachs'  curvature  : 


CHAP.  III.       OF  THE  KADICLE   OF  THE   BEAN.  149 

in  the  fourth  case  alone  no  reason  could  be  assigned 
why  the  radicle  had  not  been  at  all  deflected.  These 
experiments  suffice  to  prove  that  the  apex  of  the 
radicle  possesses  the  extraordinary  power  of  discri- 
minating between  thin  card  and  very  thin  paper,  and 
is  deflected  from  the  side  pressed  by  the  more  re- 
sisting or  harder  substance. 

Some  trials  were .  next  made  by  irritating  the  tips 
without  any  object  being  left  in  contact  with  them. 
Nine  radicles,  suspended  over  water,  had  their  tips 
rubbed,  each  six  times  with  a  needle,  with  sufficient 
force  to  shake  the  whole  bean ;  the  temperature  was 
favourable,  viz.  about  63°  F.  In  7  out  of  these  cases 
no  effect  whatever  was  produced ;  in  the  eighth  case 
the  radicle  became  slightly  deflected  from,  and  in  the 
ninth  case  slightly  deflected  towards,  the  rubbed  side  ; 
but  these  two  latter  opposed  curvatures  were  probably 
accidental,  as  radicles  do  not  always  grow  perfectly 
straight  downwards.  The  tips  of  two  other  radicles 
were  rubbed  in  the  same  manner  for  15  seconds  with 
a  little  round  twig,  two  others  for  30  seconds,  and  two 
others  for  1  minute,  but  without  any  effect  being  pro- 
duced. We  may  therefore  conclude  from  these  15 
trials  that  the  radicles  are  not  sensitive  to  temporary 
contact,  but  are  acted  on  only  by  prolonged,  though 
very  slight,  pressure. 

We  then  tried  the  effects  of  cutting  off  a  very  thin 
slice  parallel  to  one  of  the  sloping  sides  of  the  apex, 
as  we  thought  that  the  wound  would  cause  prolonged 
irritation,  which  might  induce  bending  towards  the 
opposite  side,  as  in  the  case  of  an  attached  object. 
Two  preliminary  trials  were  made :  firstly,  slices  were 
cut  from  the  radicles  of  6  beans  suspended  in  damp 
air,  with  a  pair  of  scissors,  which,  though  sharp, 
probably  caused  considerable  crushing,  and  no  curva- 


150  SENSITIVENESS   OF  THE  APEX        CHAP.  III. 

ture  followed.  Secondly,  thin  slices  were .  cut  with  a 
razor  obliquely  off  the  tips  of  three  radicles  similarly 
suspended;  and  after  44  h.  two  were  found  plainly 
bent  from  the  sliced  surface  ;  and  the  third,  the  whole 
apex  of  which  had  been  cut  off  obliquely  by  accident, 
was  curled  upwards  over  the  bean,  but  it  was  not 
clearly  ascertained  whether  the  curvature  had  been  at 
first  directed  from  the  cut  surface.  These  results  led 
us  to  pursue  the  experiment,  and  18  radicles,  which 
had  grown  vertically  downwards  in  damp  air,  had  one 
side  of  their  conical  tips  sliced  off  with  a  razor.  The 
tips  were  allowed  just  to  enter  the  water  in  the  jars, 
and  they  were  exposed  to  a  temperature  14°-16°  C. 
(57°-61°  F.).  The  observations  were  made  at  dif- 
ferent times.  Three  were  examined  12  h.  after  being 
sliced,  and  were  all  slightly  curved  from  the  cut 
surface;  and  the  curvature  increased  considerably  after 
an  additional  12  h.  Eight  were  examined  after  19  h. ; 
four  after  22  h.  30  m. ;  and  three  after  25  h.  The 
final  result  was  that  out  of  the  18  radicles  thus  tried, 
13  were  plainly  bent  from  the  cut  surface  after  the 
above  intervals  of  time ;  and  one  other  became  so 
after  an  additional  interval  of  13  h.  30  m.  So  that 
only  4  out  of  the  18  radicles  were  not  acted  on.  To 
these  18  cases  the  3  previously  mentioned  ones  should 
be  added.  It  may,  therefore,  be  concluded  that  a  thin 
slice  removed  by  a  razor  from  one  side  of  the  conical 
apex  of  the  radicle  causes  irritation,  like  that  from  an 
attached  object,  and  induces  curvature  from  the  injured 
surface. 

Lastly,  dry  caustic  (nitrate  of  silver)  was  employed 
to  irritate  one  side  of  the  apex.  If  one  side  of  the 
apex  or  of  the  whole  terminal  growing  part  of  a 
radicle,  is  by  any  means  killed  or  badly  injured,  the 
other  side  continues  to  grow ;  and  this  causes  the  part 


CHAP.  III.       OF   THE   RADICLE   OF  THE   BEAN.  151 

to  bend  over  towards  the  injured  side.*  But  in  the 
following  experiments  we  endeavoured,  generally  with 
success,  to  irritate  the  tips  on  one  side,  without  badly 
injuring  them.  This  was  effected  by  first  drying  the 
tip  as  far  as  possible  with  blotting-paper,  though  it  still 
remained  somewhat  damp,  and  then  touching  it  once 
with  quite  dry  caustic.  Seventeen  radicles  were  thus 
treated,  and  were  suspended  in  moist  air  over  water  at 
a  temperature  of  58 J  F.  They  were  examined  after 
an  interval  of  21  h.  or  24  h.  The  tips  of  two  were 
found  blackened  equally  all  round,  so  that  they  could 
tell  nothing  and  were  rejected,  15  being  left.  Of 
these,  10  were  curved  from  the  side  which  had  been 
touched,  where  there  was  a  minute  brown  or  blackish 
mark.  Five  of  these  radicles,  three  of  which  were 
already  slightly  deflected,  were  allowed  to  enter  the 
water  in  the  jar,  and  were  re-examined  after  an  addi- 
tional interval  of  27  h.  (i.e.  in  48  h.  after  the  appli- 
cation of  the  caustic),  and  now  four  of  them  had 
become  hooked,  being  bent  from  the  discoloured  side 
with  their  points  directed  to  the  zenith ;  the  fifth 
remained  unaffected  and  straight.  Thus  11  radicles 
out  of  the  15  were  acted  on.  But  the  curvature  of 
the  four  just  described  was  so  plain,  that  they  alone 
would  have  sufficed  to  show  that  the  radicles  of  the 
bean  bend  away  from  that  side  of  the  apex  which  has 
been  slightly  irritated  by  caustic. 

The  power  of  an  Irritant  on  the  apex  of  the  Radicle 


*  Ciesielski  found  this  to  be  the  pended  over  water,  with  a  thick 

case  ('  Untersuchungen  tiber  die  layer  of  grease,  which  is  very 

Abwartskriimmung  der  Wurzel,'  injurious  or  even  fatal  to  grow- 

1871,  p.  28)  after  burning  with  ing    parts;    for    after   48    hours 

heated  platinum  one  side  of  a  five  of  these  radicles  were  curved 

radicle.     So    did  we    when  we  towards  the    greased  side,   two 

painted  longitudinally  half  of  the  remaining  straight, 
whole  length  of  7  radicles,  sus- 


152  SENSITIVENESS  OF  THE  APEX        CHAP.  III. 

of  the  Sean,  compared  uith  that  of  Geotropism. — We 
know  that  when  a  little  square  of  card  or  other 
object  is  fixed  to  one  side  of  the  tip  of  a  vertically 
dependent  radicle,  the  growing  part  bends  from  it 
often  into  a  semicircle,  in  opposition  to  geotropism, 
which  force  is  conquered  by  the  effect  of  the  irri- 
tation from  the  attached  object.  Kadicles  were  there- 
fore extended  horizontally  in  damp  air,  kept  at 
the  proper  low  temperature  for  full  sensitiveness, 
and  squares  of  card  were  affixed  with  shellac  on  the 
lower  sides  of  their  tips,  so  that  if  the  squares 
acted,  the  terminal  growing  part  would  curve  upwards. 
Firstly,  eight  beans  were  so  placed  that  their  short, 
young,  horizontally  extended  radicles  would  be  simul- 
taneously acted  on  both  by  geotropism  and  by  Sachs' 
curvature,  if  the  latter  came  into  play ;  and  they  all 
eight  became  bowed  downwards  to  the  centre  of  the 
earth  in  20  h.,  excepting  one  which  was  only  slightly 
acted  on.  Two  of  them  were  a  little  bowed  downwards 
in  only  5  h. !  Therefore  the  cards,  affixed  to  the  lower 
sides  of  their  tips,  seemed  to  produce  no  effect ;  and 
geotropism  easily  conquered  the  effects  of  the  irritation 
thus  caused.  Secondly,  5  oldish  radicles,  1£  inch  in 
length,  and  therefore  less  sensitive  than  the  above- 
mentioned  young  ones,  were  similarly  placed  and 
similarly  treated.  From  what  has  been  seen  on  many 
other  occasions,  it  may  be  safely  inferred  that  if  they 
had  been  suspended  vertically  they  would  have  bent 
away  from  the  cards ;  and  if  they  had  been  extended 
horizontally,  without  cards  attached  to  them,  they 
would  have  quickly  bent  vertically  downwards  through 
geotropism;  but  the  result  was  that  two  of  these 
radicles  were  still  horizontal  after  23  h. ;  two  were 
curved  only  slightly,  and  the  fifth  as  much  as  40° 
beneath  the  horizon.  Thirdly,  5  beans  were  fastened 


CHAP.  III.        OF  THE  EADICLE   OF  THE  BEAN.  153 

with  their  flat  surfaces  parallel  to  the  cork-lid,  so  that 
Sachs'  curvature  would  not  tend  to  make  the  hori- 
zontally extended  radicles  turn  either  upwards  or 
downwards,  and  little  squares  of  card  were  affixed  as 
before,  to  the  lower  sides  of  their  tips.  The  result 
was  that  all  five  radicles  were  bent  down,  or  towards 
the  centre  of  the  earth,  after  only  8  h.  20  m.  At 
the  same  time  and  within  the  same  jars,  3  radicles  of 
the  same  age,  with  squares  affixed  to  one  side,  were 
suspended  vertically ;  and  after  8  h.  20  m.  they  were 
considerably  deflected  from  the  cards,  and  therefore 
curved  upwards  in  opposition  to  geotropism.  In  these 
latter  cases  the  irritation  from  the  squares  had  over- 
powered geotropism ;  whilst  in  the  former  cases,  in 
which  the  radicles  were  extended  horizontally,  geo- 
tropism had  overpowered  the  irritation.  Thus  within 
the  same  jars,  some  of  the  radicles  were  curving 
upwards  and  others  downwards  at  the  same  time — 
these  opposite  movements  depending  on  whether  the 
radicles,  when  the  squares  were  first  attached  to  them, 
projected  vertically  down,  or  were  extended  horizon- 
tally. This  difference  in  their  behaviour  seems  at  first 
inexplicable,  but  can,  we  believe,  be  simply  explained 
by  the  difference  between  the  initial  power  of  the  two 
forces  under  the  above  circumstances,  combined  with 
the  well-known  principle  of  the  after-effects  of  a  sti- 
mulus. When  a  young  and  sensitive  radicle  is  extended 
horizontally,  with  a  square  attached  to  the  lower  side 
of  the  tip,  geotropism  acts  on  it  at  right  angles,  and, 
as  we  have  seen,  is  then  evidently  more  efficient  than 
the  irritation  from  the  square  ;  and  the  power  of  geo- 
tropism will  be  strengthened  at  each  successive  period 
by  its  previous  action — that  is,  by  its  after-effects. 
On  the  other  hand,  when  a  square  is  affixed  to  a 
vertically  dependent  radicle,  and  the  apex  begins  to 


154  SENSITIVENESS  OF   THE  KADICLE.       CHAP.  III. 

curve  upwards,  this  movement  will  be  opposed  by  geo- 
tropism  acting  only  at  a  very  oblique  angle,  and  the 
irritation  from  the  card  will  be  strengthened  by  its 
previous  action.  We  may  therefore  conclude  that  the 
initial  power  of  an  irritant  on  the  apex  of  the  radicle 
of  the  bean,  is  less  than  that  of  geotropism  when 
acting  at  right  angles,  but  greater  than  that  of  geo- 
tropism when  acting  obliquely  on  it. 

Sensitiveness  of  the  tips  of  the  Secondary  Radicles  of  the 
Sean  to  contact. — All  the  previous  observations  relate 
to  the  main  or  primary  radicle.  Some  beans  suspended 
to  cork-lids,  with  their  radicles  dipping  into  water,  had 
developed  secondary  or  lateral  radicles,  which  were 
afterwards  kept  in  very  damp  air,  at  the  proper  low 
temperature  for  full  sensitiveness.  They  projected, 
as  usual,  almost  horizontally,  with  only  a  slight 
downward  curvature,  and  retained  this  position 
during  several  days.  Sachs  has  shown*  that  these 
secondary  roots  are  acted  on  in  a  peculiar  manner  by 
geotropism,  so  that  if  displaced  they  reassume  their 
former  sub-horizontal  position,  and  do  not  bend  verti- 
cally downwards  like  the  primary  radicle.  Minute 
squares  of  the  stiff  sanded  paper  were  affixed  by 
means  of  shellac  (but  in  some  instances  with  thick 
gum-water)  to  the  tips  of  39  secondary  radicles  of 
different  ages,  generally  the  uppermost  ones.  Most 
of  the  squares  were  fixed  to  the  lower  sides  of  the  apex, 
so  that  if  they  acted  the  radicle  would  bend  upwards  ; 
but  some  were  fixed  laterally,  and  a  few  on  the  upper 
side.  Owing  to  the  extreme  tenuity  of  these  radicles, 
it  was  very  difficult  to  attach  the  square  to  the 
actual  apex.  Whether  owing  to  this  or  some  other 
circumstance,  only  nine  of  the  squares  induced  any 


*  '  Arbeiten  Eot.  Inst.,  Wiirzburg,'  Heft  iv.  1874,  p.  605-617. 


CHAP.  III.       SENSITIVENESS   OF   THE   RADICLE.  155 

curvature.  The  curvature  amounted  in  some  cases  to 
about  45°  above  the  horizon,  in  others  to  90°,  and  then 
the  tip  pointed  to  the  zenith.  In  one  instance  a 
distinct  upward  curvature  was  observed  in  8  h.  15  m., 
but  usually  not  until  24  h.  had  elapsed.  Although 
only  9  out  of  39  radicles  were  affected,  yet  the  curva- 
ture was  so  distinct  in  several  of  them,  that  there  could 
be  no  doubt  that  the  tip  is  sensitive  to  slight  contact, 
and  that  the  growing  part  bends  away  from  the  touch- 
ing object.  It  is  possible  that  some  secondary  radicles 
are  more  sensitive  than  others  ;  for  Sachs  has  proved  * 
the  interesting  fact  that  each  individual  secondary 
radicle  possesses  its  own  peculiar  constitution. 

Sensitiveness  to  contact  of  the  Primary  Eadicle,  a  little 
above  the  apex,  in  the  Bean  ( Vicia  faba)  and  Pea  (Pisum 
sativnm). — The  sensitiveness  of  the  apex  of  the  radicle 
in  the  previously  described  cases,  and  the  consequent 
curvature  of  the  upper  part  from  the  touching  object 
or  other  source  of  irritation,  is  the  more  remarkable, 
because  8achs  f  has  shown  that  pressure  at  the  distance 
of  a  few  millimeters  above  the  apex  causes  the  radicle 
to  bend,  like  a  tendril,  towards  the  touching  object. 
By  fixing  pins  so  that  they  pressed  against  the  radicles 
of  beans  suspended  vertically  in  damp  air,  we  saw  this 
kind  of  curvature  ;  but  rubbing  the  part  with  a  twig 
or  needle  for  a  few  minutes  produced  no  effect.  Haber- 
landt  remarks,:]:  that  these  radicles  in  breaking  through 
the  seed-coats  often  rub  and  press  against  the  ruptured 
edges,  and  consequently  bend  round  them.  As  little 
squares  of  the  card-like  paper  affixed  with  shellac  to 
the  tips  were  highly  efficient  in  causing  the  radicles 
to  bend  away  from  them,  similar  pieces  (of  about  -^th 


*  '  Arbeiten  Bot.  Instit.,  Wiirz-  J  '  Die  Schutzeinrichtungen  der 

burg,'  Heft.  iv.  1874,  p.  620.  Keimpflanze,'  1877,  p.  25. 

f  Ibid.  Heft  iii.  1873,  p.  437. 


156  SENSITIVENESS   OF  THE  CHAP.  III. 

inch  square,  or  rather  less)  were  attached  in  the  same 
manner  to  one  side  of  the  radicle  at  a  distance  of  3  or 
4  mm.  above  the  apex.  In  our  first  trial  on  15  radicles 
no  effect  was  produced.  In  a  second  trial  on  the  same 
number,  three  became  abruptly  curved  (but  only  one 
strongly)  towards  the  card  within  24  h.  From  these 
cases  we  may  infer  that  the  pressure  from  a  bit  of  card 
affixed  with  shellac  to  one  side  above  the  apex,  is  hardly 
a  sufficient  irritant ;  but  that  it  occasionally  causes  the 
radicle  to  bend  like  a  tendril  towards  this  side. 

We  next  tried  the  effect  of  rubbing  several  radicles 
at  a  distance  of  4  mm.  from  the  apex  for  a  few  seconds 
with  lunar  caustic  (nitrate  of  silver)  ;  and  although  the 
radicles  had  been  wiped  dry  and  the  stick  of  caustic 
was  dry,  yet  the  part  rubbed  was  much  injured  and  a 
slight  permanent  depression  was  left.  In  such  cases 
the  opposite  side  continues  to  grow,  and  the  radicle 
necessarily  becomes  bent  towards  the  injured  side. 
But  when  a  point  4  mm.  from  the  apex  was  momen- 
tarily touched  with  dry  caustic,  it  was  only  faintly 
discoloured,  and  no  permanent  injury  was  caused.  This 
was  shown  by  several  radicles  thus  treated  straighten- 
ing themselves  after  one  or  two  days ;  yet  at  first  they 
became  curved  towards  the  touched  side,  as  if  they  had 
been  there  subjected  to  slight  continued  pressure. 
These  cases  deserve  notice,  because  when  one  side  of 
the  apex  was  just  touched  with  caustic,  the  radicle,  as 
we  have  seen,  curved  itself  in  an  opposite  direction,  that 
is,  away  from  the  touched  side. 

The  radicle  of  the  common  pea  at  a  point  a  little 
above  the  apex  is  rather  more  sensitive  to  continued 
pressure  than  that  of  the  bean,  and  bends  towards  the 
pressed  side.*  We  experimented  on  a  variety  (York- 


Sachs,  «  Aibeiten  Bot.  lustitut.,  Wurzburg,'  Heft  iii.  p.  438. 


CHAP.  III.        UPPER  PART  OF  THE  RADICLE.  15? 

shire  Hero)  which  has  a  much  wrinkled  tough  skin, 
too  large  for  the  included  cotyledons ;  so  that  out  of 
30  peas  which  had  been  soaked  for  24  h.  and  allowed 
to  germinate  on  damp  sand,  the  radicles  of  three  were 
unable  to  escape,  and  were  crumpled  up  in  a  strange 
manner  within  the  skin ;  four  other  radicles  were 
abruptly  bent  round  the  edges  of  the  ruptured  skin 
against  which  they  had  pressed.  Such  abnormalities 
would  probably  never,  or  very  rarely,  occur  with  forms 
developed  in  a  state  of  nature  and  subjected  to  natural 
selection.  One  of  the  four  radicles  just  mentioned  in 
doubling  backwards  came  into  contact  with  the  pin 
by  which  the  pea  was  fixed  to  the  cork-lid  ;  and  now  it 
bent  at  right  angles  round  the  pin,  in  a  direction  quite 
different  from  that  of  the  first  curvature  due  to  contact 
with  the  ruptured  skin ;  and  it  thus  afforded  a  good 
illustration  of  the  tendril-like  sensitiveness  of  the 
radicle  a  little  above  the  apex. 

Little  squares  of  the  card-like  paper  were  next 
affixed  to  radicles  of  the  pea  at  4  mm.  above  the  apex, 
in  the  same  manner  as  with  the  bean.  Twenty-eight 
radicles  suspended  vertically  over  water  were  thus 
treated  on  different  occasions,  and  13  of  them  became 
curved  towards  the  cards.  The  greatest  degree  of 
curvature  amounted  to  62°  from  the  perpendicular; 
but  so  large  an  angle  was  only  once  formed.  On  one 
occasion  a  slight  curvature  was  perceptible  after  5  h. 
45  m.,  and  it  was  generally  well-marked  after  14  h. 
There  can  therefore  be  no  doubt  that  with  the  pea, 
irritation  from  a  bit  of  card  attached  to  one  side  of  the 
radicle  above  the  apex  suffices  to  induce  curvature. 

Squares  of  card  were  attached  to  one  side  of  the  tips 
of  11  radicles  within  the  same  jars  in  which  the  above 
trials  were  made,  and  five  of  them  became  plainly, 
and  one  slightly,  curved  away  from  this  side.  Other 


158  SENSITIVENESS   OF  THE  APEX        CHAP.  III. 

analogous  cases  will  be  immediately  described.  The 
fact  is  here  mentioned  because  it  was  a  striking  spec- 
tacle, showing  the  difference  in  the  sensitiveness  of 
the  radicle  in  different  parts,  to  behold  in  the  same 
jar  one  set  of  radicles  curved  away  from  the  squares  on 
their  tips,  and  another  set  curved  towards  the  squares 
attached  a  little  higher  up.  Moreover,  the  kind  of 
curvature  in  the  two  cases  is  different.  The  squares 
attached  above  the  apex  cause  the  radicle  to  bend 
abruptly,  the  part  above  and  beneath  remaining  nearly 
straight ;  so  that  here  there  is  little  or  no  transmitted 
effect.  On  the  other  hand,  the  squares  attached  to 
the  apex  affect  the  radicle  for  a  length  of  from  about 
4  to  even  8  mm.,  inducing  in  most  cases  a  sym- 
metrical curvature ;  so  that  here  some  influence  is 
transmitted  from  the  apex  for  this  distance  along  the 
radicle. 

Pisum  sativum  (var.  Yorkshire  Hero)  :  Sensitiveness  of 
the  apex  of  the  Radicle. — Little  squares  of  the  same  card- 
like  paper  were  affixed  (April  24th)  with  shellac  to 
one  side  of  the  apex  of  10  vertically  suspended  radicles  : 
the  temperature  of  the  water  in  the  bottom  of  the  jars 
was  60°-61°  F.  Most  of  these  radicles  were  acted  on 
in  8  h.  30  m. ;  and  eight  of  them  became  in  the  course 
of  24  h.  conspicuously,  and  the  remaining  two  slightly, 
deflected  from  the  perpendicular  and  from  the  side 
bearing  the  attached  squares.  Thus  all  were  acted  on  ; 
but  it  will  suffice  to  describe  two  conspicuous  cases. 
In  one  the  terminal  portion  of  the  radicle  was  bent  at 
right  angles  (A,  Fig.  66)  after  24  h. ;  and  in  the  other 
(B)  it  had  by  this  time  become  hooked,  with  the  apex 
pointing  to  the  zenith.  The  two  bits  of  card  here  used 
were  '07  inch  in  length  and  '04  inch  in  breadth.  Two 
other  radicles,  which  after  8  h.  30  m.  were  moderately 
deflected,  became  straight  again  after  24  h.  Another 


CHAP.  III.          OF   THE  RADICLE  OF  THE  TEA. 


159 


trial  was  made  in  the  same  manner  with  15  radicles ; 
but  from  circumstances,  not  worth  explaining,  they 
were  only  once  and  briefly  examined  after  the  short 

Fig  66. 


B. 

Pisum  sativum :  deflection  produced  within  24  hours  in  the  growth  of 
vertically  dependent  radicles,  by  little  squares  of  card  affixed  with 
shellac  to  one  side  of  apex  :  A,  bent  at  right  angles  ;  B,  hooked. 

interval  of  5  h.  30  m. ;  and  we  merely  record  in  our 
notes  "  almost  all  bent  slightly  from  the  perpendicular, 
and  away  from  the  squares ;  the  deflection  amounting 
in  one  or  two  instances  to  nearly  a  rectangle."  These 
two  sets  of  cases,  especially  the  first  one,  prove  that 
the  apex  of  the  radicle  is  sensitive  to  slight  contact 
and  that  the  upper  part  bends  from  the  touching 
object.  Nevertheless,  on  June  1st  and  4th,  8  other 
radicles  were  tried  in  the  same  manner  at  a  tempera- 
ture of  58°-60°  F.,  and  after  24  h.  only  1  was  decidedly 
bent  from  the  card,  4  slightly,  2  doubtfully,  and  1  not 
in  the  least.  The  amount  of  curvature  was  unaccount- 
ably small ;  but  all  the  radicles  which  were  at  all  bent, 
were  bent  away  from  the  cards. 

We  now  tried  the  effects  of  widely  different  tempera- 
tures on  the  sensitiveness  of  these  radicles  with  squares 
8 


160  SENSITIVENESS   OF   THE   APEX        CHAP.  III. 

of  card  attached  to  their  tips.  Firstly,  13  peas,  most 
of  them  having  very  short  and  young  radicles,  were 
placed  in  an  ice-box,  in  which  the  temperature  rose 
during  three  days  from  44°  to  47°  F.  They  grew  slowly, 
but  10  out  of  the  13  became  in  the  course  of  the  three 
days  very  slightly  curved  from  the  squares ;  the  other 
3  were  not  aifected ;  so  that  this  temperature  was  too 
low  for  any  high  degree  of  sensitiveness  or  for  much 
movement.  Jars  with  13  other  radicles  were  next 
placed  on  a  chimney-piece,  where  they  were  subjected 
to  a  temperature  of  between  68°  and  72°  F.,  and 
after  24  h.,  4  were  conspicuously  curved  from  the 
cards,  2  slightly,  and  7  not  at  all ;  so  that  this  tem- 
perature was  rather  too  high.  Lastly,  12  radicles 
were  subjected  to  a  temperature  varying  between 
72°  and  85°  F.,  and  none  of  them  were  in  the  least 
affected  by  the  squares.  The  above  several  trials, 
especially  the  first  recorded  one,  indicate  that  the 
most  favourable  temperature  for  the  sensitiveness  of 
the  radicle  of  the  pea  is  about  60°  F. 

The  tips  of  6  vertically  dependent  radicles  were 
touched  once  with  dry  caustic,  in  the  manner  described 
under  Vicia  fdba.  After  24  h.  four  of  them  were  bent 
from  the  side  bearing  a  minute  black  mark ;  and  the 
curvature  increased  in  one  case  after  38  h.,  and  in 
another  case  after  48  h.,  until  the  terminal  part  pro- 
jected almost  horizontally.  The  two  remaining  ra- 
dicles were  not  affected. 

With  radicles  of  the  bean,  when  extended  horizontally 
in  damp  air,  geotropism  always  conquered  the  effects 
of  the  irritation  caused  by  squares  of  card  attached  to 
the  lower  sides  of  their  tips.  A  similar  experiment 
was  tried  on  13  radicles  of.the  pea;  the  squares  being 
attached  with  shellac,  and  the  temperature  between 
58°-60°  F.  The  result  'was  somewhat  different ;  for 


CHAP.  III.          OF   THE  KADICLE   OF  THE  PEA.  161 

these  radicles  are  either  less  strongly  acted  on  by 
geotropism,  or,  what  is  more  probable,  are  more  sen- 
sitive to  contact.  After  a  time  geotropism  always 
prevailed,  but  its  action  was  often  delayed;  and  in 
three  instances  there  was  a  most  curious  struggle 
between  geotropism  and  the  irritation  caused  by  the 
cards.  Four  of  the  13  radicles  were  a  little  curved 
downwards  within  6  or  8  h.,  always  reckoning  from 
the  time  when  the  squares  were  first  attached,  and 
after  23  h.  three  of  them  pointed  vertically  down- 
wards, and  the  fourth  at  an  angle  of  45°  beneath  the 
horizon.  These  four  radicles  therefore  did  not  seem 

Fig.  67. 


Pisum  sativum:  a  radicle  extended  horizontally  in  damp  air  with  a  little 
square  of  card  affixed  to  the  lower  side  of  its  tip,  causing  it  to  bend 
upwards  in  opposition  to  geotropism.  The  deflection  of  the  radicle 
after  21  hours  is  shown  at  A,  and  of  the  same  radicle  after  45  hours  at 
B,  now  forming  a  loop. 

to  have  been  at  all  affected  by  the  attached  squares. 
Four  others  were  not  acted  on  by  geotropism  within 
the  first  6  or  8  h.,  but  after  23  h.  were  much  bowed 
down.  Two  others  remained  almost  horizontal  for 
23  h.,  but  afterwards  were  acted  on.  So  that  in  these 
latter  six  cases  the  action  of  geotropism  was  much 
delayed.  The  eleventh  radicle  was  slightly  curved 
down  after  8  h.,  but  when  looked  at  again  after  23  h. 
the  terminal  portion  was  curved  upwards;  if  it  had 


162  SENSITIVENESS   OF   THE  APEX        CHAP.  lit, 

been  longer  observed,  the  tip  no  doubt  would  have 
been  found  again  curved  down,  and  it  would  have 
formed  a  loop  as  in  the  following  case.  The  twelfth 
radicle  after  6  h.  was  slightly  curved  downwards ;  but 
when  looked  at  again  after  21  h.,  this  curvature  had 
disappeared  and  the  apex  pointed  upwards ;  after  30  h. 
the  Tadicle  formed  a  hook,  as  shown  at  A  (Fig.  67) ; 
which  hook  after  45  h.  was  converted  into  a  loop  (B). 
The  thirteenth  radicle  after  6  h.  was  slightly  curved 
downwards,  but  within  21  h.  had  curved  considerably 
up,  and  then  down  again  at  an  angle  of  45°  beneath 
the  horizon,  afterwards  becoming  perpendicular.  In 
these  three  last  cases  geotropism  and  the  irritation 
caused  by  the  attached  squares  alternately  prevailed 
in  a  highly  remarkable  manner;  geotropism  being 
ultimately  victorious. 

Similar  experiments  were  not  always  quite  so  suc- 
cessful as  in  the  above  cases.  Thus  6  radicles,  horizon- 
tally extended  with  attached  squares,  were  tried  on 
June  8th  at  a  proper  temperature,  and  after  7  h.  30  m. 
none  were  in  the  least  curved  upwards  and  none  were 
distinctly  geotropic ;  whereas  of  6  radicles  without  any 
attached  squares,  which  served  as  standards  of  com- 
parison or  controls,  3  became  slightly  and  3  almost 
rectangularly  geotropic  within  the  7h.  30m.;  but 
after  23  h.  the  two  lots  were  equally  geotropic.  On 
July  10th  another  trial  was  made  with  6  horizontally 
extended  radicles,  with  squares  attached  in  the  same 
manner  beneath  their  tips ;  and  after  7  h.  30  m.,  4  were 
slightly  geotropic,  1  remained  horizontal,  and  1  was 
curved  upwards  in  opposition  to  gravity  or  geotropism. 
This  latter  radicle  after  48  h.  formed  a  loop,  like  that 
at  B  (Fig.  67). 

An  analogous  trial  was  now  made,  but  instead  of 
attaching  squares  of  card  to  the  lower  sides  of  the 


CHAP.  III.       OF  THE   KADICLE   OF  PHASEOLUS.  163 

tips,  these  were  touched  with  dry  caustic.  The  details 
of  the  experiment  will  be  given  in  the  chapter  on 
Geotropism,  and  it  will  suffice  here  to  say  that  10 
peas,  with  radicles  extended  horizontally  and  not  cau- 
terised, were  laid  on  and  under  damp  friable  peat ; 
these,  which  served  as  standards  or  controls,  as  well  as 
10  others  which  had  been  touched  on  the  upper  side 
with  the  caustic,  all  became  strongly  geotropic  in  24  h. 
Nine  radicles,  similarly  placed,  had  their  tips  touched 
on  the  lower  side  with  the  caustic ;  and  after  24  h., 

3  were  slightly  geotropic,  2  remained  horizontal,  and 

4  were  bowed  upwards  in  opposition  to  gravity  and  to 
geotropism.      This   upward   curvature  was   distinctly 
visible  in  8  h.  45  m.  after  the  lower  sides  of  the  tips 
had  been  cauterised. 

Little  squares  of  card  were  affixed  with  shellac  on 
two  occasions  to  the  tips  of  22  young  and  short 
secondary  radicles,  which  had  been  emitted  from  the 
primary  radicle  whilst  growing  in  water,  but  were  now 
suspended  in  damp  air.  Besides  the  difficulty  of 
attaching  the  squares  to  such  finely  pointed  objects 
as  were  these  radicles,  the  temperature  was  too  high, 
—varying  on  the  first  occasion  from  72°  to  77°  F.,  and 
on  the  second  being  almost  steadily  78°  F. ;  and  this 
probably  lessened  the  sensitiveness  of  the  tips.  The 
result  was  that  after  an  interval  of  8  h.  30  m.,  6  of  the 
22  radicles  were  bowed  upwards  (one  of  them  greatly) 
in  opposition  to  gravity,  and  2  laterally ;  the  remain- 
ing 14  were  not  affected.  Considering  the  unfavour- 
able circumstances,  and  bearing  in  mind  the  case  of 
the  bean,  the  evidence  appears  sufficient  to  show  that 
the  tips  of  the  secondary  radicles  of  the  pea  are 
sensitive  to  slight  contact. 

Phaseolus  multiflorus :  Sensitiveness  of  the  apex  of  the 
Badicle. — Fifty-nine  radicles  were  tried  with  squares 


164  SENSITIVENESS  OF  THE  APEX         CHAP.  III. 

of  various  sizes  of  the  same  card-like  paper,  also  with 
bits  of  thin  glass  and  rough  cinders,  affixed  with  shellac 
to  one  side  of  the  apex.  Bather  large  drops  of  the 
dissolved  shellac  were  also  placed  on  them  and  allowed 
to  set  into  hard  beads.  The  specimens  were  subjected 
to  various  temperatures  between  60°  and  72°  F.,  more 
commonly  at  about  the  latter.  But  out  of  this  con- 
siderable number  of  trials  only  5  radicles  were  plainly 
bent,  and  8  others  slightly  or  even  doubtfully,  from 
the  attached  objects ;  the  remaining  46  not  being  at 
all  affected.  It  is  therefore  clear  that  the  tips  of  the 
radicles  of  this  Phaseolus  are  much  less  sensitive  to 
contact  than  are  those  of  the  bean  or  pea.  We 
thought  that  they  might  be  sensitive  to  harder 
pressure,  but  after  several  trials  we  could  not  devise 
any  method  for  pressing  harder  on  one  side  of  the 
apex  than  on  the  other,  without  at  the  same  time 
offering  mechanical  resistance  to  its  growth.  We 
therefore  tried  other  irritants. 

The  tips  of  13  radicles,  dried  with  blotting-paper, 
were  thrice  touched  or  just  rubbed  on  one  side 
with  dry  nitrate  of  silver.  They  were  rubbed  thrice, 
because  we  supposed  from  the  foregoing  trials,  that 
the  tips  were  not  highly  sensitive.  After  24  h.  the 
tips  were  found  greatly  blackened ;  6  were  blackened 
equally  all  round,  so  that  no  curvature  to  any  one 
side  could  be  expected;  6  were  much  blackened  on 
one  side  for  a  length  of  about  -Vth  of  an  inch,  and 
this  length  became  curved  at  right  angles  towards  the 
blackened  surface,  the  curvature  afterwards  increasing 
in  several  instances  until  little  hooks  were  formed. 
It  was  manifest  that  the  blackened  side  was  so  much 
injured  that  it  could  not  grow,  whilst  the  opposite 
side  continued  to  grow.  One  alone  out  of  these  13 
radicles  became  curved  from  the  blackened  side,  the 


CHAP.  III.       OF  THE  KADICLE   OF   PHASEOLUS.  165 

curvature  extending   for   some   little  distance   above 
the  apex. 

After  the  experience  thus  gained,  the  tips  of  six 
almost  dry  radicles  were  once  touched  with  the  dry 
caustic  on  one  side ;  and  after  an  interval  of  10  m. 
were  allowed  to  enter  water,  which  was  kept  at  a 
temperature  of  65°-67°  F.  The  result  was  that  after 
an  interval  of  8  h.  a  minute  blackish  speck  could 
just  be  distinguished  on  one  side  of  the  apex  of  five 
of  these  radicles,  all  of  which  became  curved  towards 
the  opposite  side — in  two  cases  at  about  an  angle 
of  45° — in  two  other  cases  at  nearly  a  rectangle — and 
in  the  fifth  case  at  above  a  rectangle,  so  that  the  apex 
was  a  little  hooked ;  in  this  latter  case  the  black  mark 
was  rather  larger  than  in  the  others.  After  24  h. 
from  the  application  of  the  caustic,  the  curvature  of 
three  of  these  radicles  (including  the  hooked  one)  had 
diminished ;  in  the  fourth  it  remained  the  same,  and 
in  the  fifth  it  had  increased,  the  tip  being  now  hooked. 
It  has  been  said  that  after  8  h.  black  specks  could 
be  seen  on  one  side  of  the  apex  of  five  of  the  six 
radicles  ;  on  the  sixth  the  speck,  which  was  extremely 
minute,  was  on  the  actual  apex  and  therefore  central ; 
and  this  radicle  alone  did  not  become  curved.  It  was 
therefore  again  touched  on  one  side  with  caustic,  and 
after  15  h.  30  m.  was  found  curved  from  the  perpen- 
dicular and  from  the  blackened  side  at  an  angle  of  34°, 
which  increased  in  nine  additional  hours  to  54°. 

It  is  therefore  certain  that  the  apex  of  the  radicle 
of  this  Phaseolus  is  extremely  sensitive  to  caustic, 
more  so  than  that  of  the  bean,  though  the  latter  is 
far  more  sensitive  to  pressure.  In  the  experiments 
just  given,  the  curvature  from  the  slightly  cauterised 
side  of  the  tip,  extended  along  the  radicle  for  a 
length  of  nearly  10  mm. ;  whereas  in  the  first  set 


166  SENSITIVENESS   OF  THE  APEX         CHAP.  III. 

of  experiments,  when  the  tips  of  several  were  greatly 
blackened  and  injured  on  one  side,  so  that  their  growth 
was  arrested,  a  length  of  less  than  3  mm.  became 
curved  towards  the  much  blackened  side,  owing  to  the 
continued  growth  of  the  opposite  side.  This  differ- 
ence in  the  results  is  interesting,  for  it  shows  that  too 
strong  an  irritant  does  not  induce  any  transmitted 
effect,  and  does  not  cause  the  adjoining,  upper  and 
growing  part  of  the  radicle  to  bend.  We  have  analo- 
gous cases  with  Drosera,  for  a  strong  solution  of  car- 
bonate of  ammonia  when  absorbed  by  the  glands,  or 
too  great  heat  suddenly  applied  to  them,  or  crushing 
them,  does  not  cause  the  basal  part  of  the  tentacles 
to  bend,  whilst  a  weak  solution  of  the  carbonate,  or  a 
moderate  heat,  or  slight  pressure  always  induces  such 
bending.  Similar  results  were  observed  with  Dionsea 
and  Pinguicula. 

The  effect  of  cutting  off  with  a  razor  a  thin  slice 
from  one  side  of  the  conical  apex  of  14  young  and 
short  radicles  was  next  tried.  Six  of  them  after  being 
operated  on  were  suspended  in  damp  air ;  the  tips  of 
the  other  eight,  similarly  suspended,  were  allowed  to 
enter  water  at  a  temperature  of  about  65°  F.  It  was 
recorded  in  each  case  which  side  of  the  apex  had 
been  sliced  off,  and  when  they  were  afterwards 
examined  the  direction  of  the  curvature  was  noted, 
before  the  record  was  consulted.  Of  the  six  radicles 
in  damp  air,  three  had  their  tips  curved  after  an 
interval  of  10  h.  15  m.  directly  away  from  the  sliced 
surface,  whilst  the  other  three  were  not  affected  and 
remained  straight ;  nevertheless,  one  of  them  after 
13  additional  hours  became  slightly  curved  from  the 
sliced  surface.  Of  the  eight  radicles  with  their  tips 
immersed  in  water,  seven  were  plainly  curved  away 
from  the  sliced  surfaces  after  10  h.  15  m. ;  and  with 


CHAP.  III.       OF  THE  KADICLE   OF  TKOP^EOLUM.         167 

respect  to  the  eighth  which  remained  quite  straight, 
too  thick  a  slice  had  been  accidentally  removed,  so 
that  it  hardly  formed  a  real  exception  to  the  general 
result.  When  the  seven  radicles  were  looked  at 
again,  after  an  interval  of  23  h.  from  the  time  of 
slicing,  two  had  become  distorted ;  four  were  deflected 
at  an  angle  of  about  70°  from  the  perpendicular  and 
from  the  cut  surface ;  and  one  was  deflected  at  nearly 
90°,  so  that  it  projected  almost  horizontally,  but  with 
the  extreme  tip  now  beginning  to  bend  downwards 
through  the  action  of  geotropism.  It  is  therefore 
manifest  that  a  thin  slice  cut  off  one  side  of  the  conical 
apex,  causes  the  upper  growing  part  of  the  radicle  of 
this  Phaseolus  to  bend,  through  the  transmitted  effects 
of  the  irritation,  away  from  the  sliced  surface. 

Trop&olum  majus:  Sensitiveness  of  the  apex  of  the 
Radicle  to  contact. — Little  squares  of  card  were  attached 
with  shellac  to  one  side  of  the  tips  of  19  radicles,  some 
of  which  were  subjected  to  78°  F.,  and  others  to  a 
much  lower  temperature.  Only  3  became  plainly 
curved  from  the  squares,  5  slightly,  4  doubtfully, 
and  7  not  at  all.  These  seeds  were,  as  we  believed, 
old,  so  we  procured  a  fresh  lot,  and  now  the  results 
were  widely  different.  Twenty-three  were  tried  in 
the  same  manner ;  five  of  the  squares  produced  no 
effect,  but  three  of  these  cases  were  no  real  exceptions, 
for  in  two  of  them  the  squares  had  slipped  and  were 
parallel  to  the  apex,  and  in  the  third  the  shellac  was 
in  excess  and  had  spread  equally  all  round  the  apex. 
One  radicle  was  deflected  only  slightly  from  the 
perpendicular  and  from  the  card ;  whilst  seventeen 
were  plainly  deflected.  The  angles  in  several  of  these 
latter  cases  varied  between  40°  and  65°  from  the 
perpendicular ;  and  in  two  of  them  it  amounted  after 
15  h.  or  16  h.  to  about  90°.  In  one  instance  a  loop 


168  SENSITIVENESS   OF  THE  APEX         CHAP.  III. 

was  nearly  completed  in  16  h.  There  can,  therefore, 
be  no  doubt  that  the  apex  is  highly  sensitive  to  slight 
contact,  and  that  the  upper  part  of  the  radicle  bends 
away  from  the  touching  object. 

Gossypium  herbaceum :  Sensitiveness  of  the  apex  of  the 
Radicle. — Kadicles  were  experimented  on  in  the  same 
manner  as  before,  but  they  proved  ill-fitted  for  our 
purpose,  as  they  soon  became  unhealthy  when  sus- 
pended in  damp  air.  Of  38  radicles  thus  suspended, 
at  temperatures  varying  from  66°  to  69°  F.,  with 
squares  of  card  attached  to  their  tips,  9  were  plainly 
and  7  slightly  or  even  doubtfully  deflected  from  the 
squares  and  from  the  perpendicular;  22  not  being 
affected.  We  thought  that  perhaps  the  above  tempera- 
ture was  not  high  enough,  so  19  radicles  with  attached 
squares,  likewise  suspended  in  damp  air,  were  subjected 
to  a  temperature  of  from  74°  to  79°  F.,  but  not  one  of 
them  was  acted  on,  and  they  soon  became  unhealthy. 
Lastly,  19  radicles  were  suspended  in  water  at  a  tem- 
perature from  70°  to  75°  F.,  with  bits  of  glass  or 
squares  of  the  card  attached  to  their  tips  by  means  of 
Canada-balsam  or  asphalte,  which  adhered  rather  better 
than  shellac  beneath  the  water.  The  radicles  did  not 
keep  healthy  for  long.  The  result  was  that  6  were 
plainly  and  2  doubtfully  deflected  from  the  attached 
objects  and  the  perpendicular ;  11  not  being  affected. 
The  evidence  consequently  is  hardly  conclusive, 
though  from  the  two  sets  of  cases  tried  under  a 
moderate  temperature,  it  is  probable  that  the  radicles 
are  sensitive  to  contact ;  and  would  be  more  so  under 
favourable  conditions. 

Fifteen  radicles  which  had  germinated  in  friable  peat 
wrere  suspended  vertically  over  water.  Seven  of  them 
served  as  controls,  and  they  remained  quite  straight 
during  24  h.  The  tips  of  the  other  eight  radicles 


CHAP.  III.       OF   THE   RADICLE   OF  CUCURBITA.  1 69 

were  just  touched  with  dry  caustic  on  one  side.  After 
only  5  h.  10  m.  five  of  them  were  slightly  curved 
from  the  perpendicular  and  from  the  side  bearing  the 
little  blackish  marks.  After  8  h.  40  m.,  4  out  of 
these  5  were  deflected  at  angles  between  15°  and  65° 
from  the  perpendicular.  On  the  other  hand,  one 
which  had  been  slightly  curved  after  5  h.  10  m.,  now 
became  straight.  After  24  h.  the  curvature  in  two 
cases  had  considerably  increased;  also  in  four  other 
cases,  but  these  latter  radicles  had  now  become  so 
contorted,  some  being  turned  upwards,  that  it  could  no 
longer  be  ascertained  -whether  they  were  still  curved 
from  the  cauterised  side.  The  control  specimens  ex- 
hibited no  such  irregular  growth,  and  the  two  sets 
presented  a  striking  contrast.  Out  of  the  8  radicles 
which  had  been  touched  with  caustic,  two  alone  were 
not  affected,  and  the  marks  left  on  their  tips  by  the 
caustic  were  extremely  minute.  These  marks  in  all 
cases  were  oval  or  elongated ;  they  were  measured  in 
three  instances,  and  found  to  be  of  nearly  the  same 
size,  viz.  §  of  a  mm.  in  length.  Bearing  this  fact  in 
mind,  it  should  be  observed  that  the  length  of  the 
curved  part  of  the  radicle,  which  had  become  deflected 
from  the  cauterised  side  in  the  course  of  8  h.  40  m., 
was  found  to  be  in  three  cases  6,  7,  and  9  mm. 

Cucurbita  ovifera :  Sensitiveness  of  the  apex  of  the  Ra- 
dicle.— The  tips  proved  ill-fitted  for  the  attachment  of 
cards,  as  they  are  extremely  fine  and  flexible.  More- 
over, owing  to  the  hypocotyls  being  soon  developed 
and  becoming  arched,  the  whole  radicle  is  quickly 
displaced  and  confusion  is  thus  caused.  A  large 
number  of  trials  were  made,  but  without  any  definite 
result,  excepting  on  two  occasions,  when  out  of  23 
radicles  10  were  deflected  from  the  attached  squares 


170  SENSITIVENESS   OF   THE   APEX         CHAP.  III. 

of  card,  and  13  were  not  acted  on.  Bather  large 
squares,  though  difficult  to  affix,  seemed  more  efficient 
than  very  small  ones. 

We  were  much  more  successful  with  caustic  ;  but  in 
our  first  trial,  15  radicles  were  too  much  cauterised, 
and  only  two  became  curved  from  the  blackened  side ; 
the  others  being  either  killed  on  one  side,  or  blackened 
equally  all  round.  In  our  next  trial  the  dried  tips 
of  11  radicles  were  touched  momentarily  with  dry 
caustic,  and  after  a  few  minutes  were  immersed  in 
water.  The  elongated  marks  thus  caused  were  never 
black,  only  brown,  and  about-  J  mm.  in  length,  or 
even  less.  In  4  h.  30  m.  after  the  cauterisation,  6  of 
them  were  plainly  curved  from  the  side  with  the 
brown  mark,  4  slightly,  and  1  not  at  all.  The  latter 
proved  unhealthy,  and  never  grew ;  and  the  marks  on 
2  of  the  4  slightly  curved  radicles  were  excessively 
minute,  one  being  distinguishable  only  with  the  aid 
of  a  lens.  Of  10  control  specimens  tried  in  the  same 
jars  at  the  same  time,  not  one  was  in  the  least  curved. 
In  8  h.  40  m.  after  the  cauterisation,  5  of  the  radicles 
out  of  the  10  (the  one  unhealthy  one  being  omitted) 
were  deflected  at  about  90°,  and  3  at  about  45°  from 
the  perpendicular  and  from  the  side  bearing  the 
brown  mark.  After  24  h.  all  10  radicles  had  in- 
creased immensely  in  length  ;  in  5  of  them  the  curva- 
ture was  nearly  the  same,  in  2  it  had  increased,  and 
in  3  it  had  decreased.  The  contrast  presented  by  the 
10  controls,  after  both  the  8  h.  40  m.  and  the  24  h. 
intervals,  was  very  great ;  for  they  had  continued  to 
grow  vertically  downwards,  excepting  two  which,  from 
some  unknown  cause,  had  become  somewhat  tortuous. 

In  the  chapter  on  G-eotropism  we  shall  see  that 
10  radicles  of  this  plant  were  extended  horizontally  on 
and  beneath  damp  friable  peat,  under  which  conditions 


CHAP.  III.         OF  THE  RADICLE   OF  EAPHANUS.  171 

they  grow  better  and  more  naturally  than  in  damp 
air ;  and  their  tips  were  slightly  cauterised  on  the 
lower  side,  brown  marks  about  ^  mm.  in  length 
being  thus  caused.  Uncauterised  specimens  similarly 
placed  became  much  bent  downwards  through  geo- 
tropism  in  the  course  of  5  or  6  hours.  After  8  h. 
only  3  of  the  cauterised  ones  were  bowed  downwards, 
and  this  in  a  slight  degree ;  4  remained  horizontal ; 
and  3  were  curved  upwards  in  opposition  to  geo- 
tropism  and  from  the  side  bearing  the  brown  mark. 
Ten  other  specimens  had  their  tips  cauterised  at  the 
same  time  and  in  the  same  degree,  on  the  upper 
side ;  and  this,  if  it  produced  any  eifect,  would  tend 
to  increase  the  power  of  geotropism ;  and  all  these 
radicles  were  strongly  bowed  downwards  after  8  h. 
From  the  several  foregoing  facts,  there  can  be  no 
doubt  that  the  cauterisation  of  the  tip  of  the  radicle 
of  this  Cucurbita  on  one  side,  if  done  lightly  enough, 
causes  the  whole  growing  part  to  bend  to  the  opposite 
side. 

Raphanus  sativus :  Sensitiveness  of  the  apex  of  the 
Radicle. — We  here  encountered  many  difficulties  in 
our  trials,  both  with  squares  of  card  and  with  caustic  ; 
for  when  seeds  were  pinned  to  a  cork-lid,  many  of  the 
radicles,  to  which  nothing  had  been  done,  grew  irre- 
gularly, often  curving  upwards,  as  if  attracted  by  the 
damp  surface  above ;  and  when  they  were  immersed 
in  water  they  likewise  often  grew  irregularly.  We 
did  not  therefore  dare  to  trust  our  experiments  with 
attached  squares  of  card ;  nevertheless  some  of  them 
seemed  to  indicate  that  the  tips  were  sensitive  to 
contact.  Our  trials  with  caustic  generally  failed  from 
the  difficulty  of  not  injuring  too  greatly  the  extremely 
fine  tips.  Out  of  7  radicles  thus  tried,  one  became 
bowed  after  22  h.  at  an  angle  of  60°,  a  second  at  40° 


172  SENSITIVENESS   OF   THE  APEX         CHAF.  III. 

and  a  third  very  slightly  from  the  perpendicular  and 
from  the  cauterised  side. 

jEsculus  hippocastanum :  Sensitiveness  of  the  apex  of 
the  Radicle. — Bits  of  glass  and  squares  of  card  were 
affixed  with  shellac  or  gum-water  to  the  tips  of  12 
radicles  of  the  horse-chestnut ;  and  when  these  objects 
fell  off,  they  were  refixed  ;  but  not  in  a  single  instance 
was  any  curvature  thus  caused.  These  massive 
radicles,  one  of  which  was  above  2  inches  in  length 
and  •  3  inch  in  diameter  at  its  base,  seemed  insensible 
to  so  slight  a  stimulus  as  any  small  attached  object. 
Nevertheless,  when  the  apex  encountered  an  obstacle 
in  its  downward  course,  the  growing  part  became  so 
uniformly  and  symmetrically  curved,  that  its  appear- 
ance indicated  not  mere  mechanical  bending,  but 
increased  growth  along  the  whole  convex  side,  due  to 
the  irritation  of  the  apex. 

That  this  is  the  correct  view  may  be  inferred  from 
the  effects  of  the  more  powerful  stimulus  of  caustic. 
The  bending  from  the  cauterised  side  occurred  much 
slower  than  in  the  previously  described  species,  and  it 
will  perhaps  be  worth  while  to  give  our  trials  in 
detail. 

The  seeds  germinated  in  sawdust,  and  one  side  of  the  tips  of 
the  radicles  were  slightly  rubbed  once  with  dry  nitrate  of  silver ; 
and  after  a  few  minutes  were  allowed  to  dip  into  water.  They 
were  subjected  to  a  rather  varying  temperature,  generally 
between  52°  and  58°  F.  A  few  cases  have  not  been  thought 
worth  recording,  in  which  the  whole  tip  was  blackened,  or  in 
which  the  seedling  soon  became  unhealthy. 

(1.)  The  radicle  was  slightly  deflected  from  the  cauterised 
side  in  one  day  (i.e.  24  h.) ;  in  three  days  it  stood  at  60°  from 
the  perpendicular ;  in  four  days  at  90°  ;  on  the  fifth  day  it  was 
curved  up  about  40°  above  the  horizon ;  so  that  it  had  passed 
through  an  angle  of  130°  in  the  five  days,  and  this  was  the 
greatest  amount  of  curvature  observed. 

(2.)  In  two  days  radicle  slightly  deflected ;  after  seven  days 


CHAP.  III.         OF  THE  RADICLE   OF   ^SCULUS.  173 

deflected  69°  from  the  perpendicular  and  from  the  cauterised 
side ;  after  eight  days  the  angle  amounted  to  nearly  90°. 

(3.)  After  one  day  slight  deflection,  but  the  cauterised  mark 
was  so  faint  that  the  same  side  was  again  touched  with  caustic. 
In  four  days  from  the  first  touch  deflection  amounted  to  78°, 
which  in  an  additional  day  increased  to  90°. 

(4.)  After  two  days  slight  deflection,  which  during  the  next 
three  days  certainly  increased  but  never  became  great;  the 
radicle  did  not  grow  well  and  died  on  the  eighth  day. 

(5.)  After  two  days  very  slight  deflection;  but  this  on  the 
fourth  day  amounted  to  56°  from  the  perpendicular  and  from 
the  cauterised  side. 

(6.)  After  three  days  doubtfully,  but  after  four  days  certainly 
deflected  from  the  cauterised  side.  On  the  fifth  day  deflection 
amounted  to  45°  from  the  perpendicular,  and  this  on  the  seventh 
day  increased  to  about  90°. 

(7.)  After  two  days  slightly  deflected ;  on  the  third  day  the 
deflection  amounted  to  25°  from  the  perpendicular,  and  this 
did  not  afterwards  increase. 

(8.)  After  one  day  deflection  distinct ;  on  the  third  day  it 
amounted  to  44°,  and  on  the  fourth  day  to  72°  from  the  perpen- 
dicular and  the  cauterised  side. 

(9.)  After  two  days  deflection  slight,  yet  distinct;  on  the 
third  day  the  tip  was  again  touched  on  the  same  side  with 
caustic  and  thus  killed. 

(10.)  After  one  day  slight  deflection,  which  after  six  days 
increased  to  50°  from  the  perpendicular  and  the  cauterised  side. 

(11.)  After  one  day  decided  deflection,  which  after  six  days 
increased  to  62°  from  the  perpendicular  and  from  the  cauterised 
side. 

(12.)  After  one  day  slight  deflection,  which  on  the  second  day 
amounted  to  35°,  on  the  fourth  day  to  50°,  and  the  sixth  day 
to  63°  from  the  perpendicular  and  the  cauterised  side. 

(13.)  Whole  tip  blackened,  but  more  on  one  side  than  the 
other ;  on  the  fourth  day  slightly,  and  on  the  sixth  day  greatly 
deflected  from  the  more  blackened  side ;  the  deflection  on  the 
ninth  day  amounted  to  90°  from  the  perpendicular. 

(14.)  Whole  tip  blackened  in  the  same  manner  as  in  the  last 
case;  on  the  second  day  decided  deflection  from  the  more 
blackened  side,  which  increased  on  the  seventh  day  to  nearly 
90° ;  on  the  following  day  the  radicle  appeared  unhealthy. 

(15.)  Here  we  had  the  anomalous  case  of  a  radicle  bending 


174  SENSITIVENESS   OF  THE  APEX         CHAP.  HI. 

slightly  towards  the  cauterised  side  on  the  first  day,  and  con- 
tinuing to  do  so  for  the  next  three  days,  when  the  deflection 
amounted  to  about  90°  from  the  perpendicular.  The  cause 
appeared  to  lie  in  the  tendril-like  sensitiveness  of  the  upper  part 
of  the  radicle,  against  which  the  point  of  a  large  triangular  flap 
of  the  seed-coats  pressed  with  considerable  force;  and  this 
irritation  apparently  conquered  that  from  the  cauterised  apex. 


These  several  cases  show  beyond  doubt  that  the 
irritation  of  one  side  of  the  apex,  excites  the  upper 
part  of  the  radicle  to  bend  slowly  towards  the  opposite 
side.  This  fact  was  well  exhibited  in  one  lot  of  five 
seeds  pinned  to  the  cork-lid  of  a  jar ;  for  when  after 
6  days  the  lid  was  turned  upside  down  and  viewed 
from  directly  above,  the  little  black  marks  made  by  the 
caustic  were  now  all  distinctly  visible  on  the  upper 
sides  of  the  tips  of  the  laterally  bowed  radicles. 

A  thin  slice  was  shaved  off  with  a  razor  from  one 
side  of  the  tips  of  22  radicles,  in  the  manner  described 
under  the  common  bean ;  but  this  kind  of  irritation 
did  not  prove  very  effective.  Only  7  out  of  the  22 
radicles  became  moderately  deflected  in  from  3  to  5 
days  from  the  sliced  surface,  and  several  of  the  others 
grew  irregularly.  The  evidence,  therefore,  is  for  from 
conclusive. 

Quercus  robur:  Sensitiveness  of  the  apex  oftlie  Radicle. 
—The  tips  of  the  radicles  of  the  common  oak  are  fully 
as  sensitive  to  slight  contact  as  are  those  of  any  plant 
examined  by  us.  They  remained  healthy  in  damp  air 
for  10  days,  but  grew  slowly.  Squares  of  the  card- 
like  paper  were  fixed  with  shellac  to  the  tips  of  15 
radicles,  and  ten  of  these  became  conspicuously  bowed 
from  the  perpendicular  and  from  the  squares  ;  two 
slightly,  and  three  not  at  all.  But  two  of  the  latter 
were  not  real  exceptions,  as  they  were  at  first  very 
short,  and  hardly  grew  afterwards.  Some  of  the  more 


CHAP.  III.       OF  THE  EADICLE  OF   QUEKCUS. 


175 


Fig.  68. 


remarkable  cases  are  worth  describing.  The  radicles 
were  examined  on  each  successive  morning,  at  nearly 
the  same  hour,  that  is,  after  intervals  of  24  h. 

No.  1.  This  radicle  suffered  from  a  series  of  accidents,  and 
acted  in  an  anomalous  manner,  for  the  apex  appeared  at  first 
insensible  arid  afterwards  sensitive  to  contact.  The  first  square 
was  attached  on  Oct.  19th  ;  on  the  21st  the 
radicle  was  not  at  all  curved,  and  the  square 
was  accidentally  knocked  off;  it  was  refixed 
on  the  22nd,  and  the  radicle  became  slightly 
curved  from  the  square,  but  the  curvature 
disappeared  on  the  23rd,  when  the  square 
was  removed  and  refixed.  No  curvature  en- 
sued, and  the  square  was  again  accidentally 
knocked  off,  and  refixed.  On  the  morning  of 
the  27th  it  was  washed  off  by  having  reached 
the  water  in  the  bottom  of  the  jar.  The 
square  was  refixed,  and  on  the  29th,  that 
is,  ten  days  after  the  first  square  had  been 
attached,  and  two  days  after  the  attachment 
of  the  last  square,  the  radicle  had  grown  to 
the  great  length  of  3'2  inches,  and  now  nuercus 
the  terminal  growing  part  had  become  bent 
away  from  the  square  into  a  hook  (see 
Fig.  68). 

No.  2.  Square  attached  on  the  19th ;  on 
the  20th  radicle  slightly  deflected  from  it 
and  from  the  perpendicular;  on  the  21st 
deflected  at  nearly  right  angles ;  it  remained  during  the  next 
two  days  in  this  position,  but  on  the  25th  the  upward  curva- 
ture was  lessened  through  the  action  of  geotropism,  and  still 
more  so  on  the  26th. 

No.  3.  Square  attached  on  the  19th ;  on  the  21st  a  trace  of 
curvature  from  the  square,  which  amounted  on  the  22nd  to 
about  4G0,  and  on  the  23rd  to  53°  from  the  perpendicular. 

No.  4.  Square  attached  on  the  21st;  on  the  22nd  trace  of 
curvature  from  the  square ;  on  the  23rd  completely  hooked 
with  the  point  turned  up  to  the  zenith.  Three  days  afterwards 
(i.e.  26th)  the  curvature  had  wholly  disappeared  and  the  apex 
pointed  perpendicularly  downwards. 

No.  5.  Square  attached  on  the  21st ;   on  the  22nd  decided 


radicle 
with  square  of  card 
attached  to  one  side 
of  apex,  causing  it 
to  become  hooked. 
Drawing  one-half 
natural  scale. 


176  SENSITIVENESS   OF   THE  APEX         CHAP.  III. 

though  slight  curvature  from  the  square ;  on  the  23rd  the  tip 
had  curved  up  above  the  hodzon,  and  on  the  2-ith  was  hooked 
with  the  apex  pointing  almost  to  the  zenith,  as  in  Fig.  68. 

No.  6.  Square  attached  on  the  2ist ;  on  the  22nd  slightly 
curved  from  the  square;  23rd  more  curved;  25th  consider- 
ably curved ;  27th  all  curvature  lost,  and  the  radicle  was  now 
directed  perpendicularly  downwards. 

No.  7.  Square  attached  on  the  21st ;  on  the  22nd  a  trace  of 
curvature  from  the  square,  which  increased  next  day,  and  on 
the  24th  amounted  to  a  right  angle. 

It  is,  therefore,  manifest  that  the  apex  of  the  radicle 
of  the  oak  is  highly  sensitive  to  contact,  and  retains 
its  sensitiveness  during  several  days.  The  movement 
thus  induced  was,  however,  slower  than  in  any  of  the 
previous  cases,  with  the  exception  of  that  of  ^Esculus. 
As  with  the  bean,  the  terminal  growing  part,  after 
bending,  sometimes  straightened  itself  through  the 
action  of  geotropism,  although  the  object  still  remained 
attached  to  the  tip. 

The  same  remarkable  experiment  was  next  tried, 
as  in  the  case  of  the  bean ;  namely,  little  squares  of 
exactly  the  same  size  of  the  card-like  sanded  paper 
and  of  very  thin  paper  (the  thicknesses  of  which  have 
been  given  under  Vicia  faba)  were  attached  with 
shellac  on  opposite  sides  (as  accurately  as  could  be 
done)  of  the  tips  of  13  radicles,  suspended  in  damp 
air,  at  a  temperature  of  65°— 66°  F.  The  result  was 
striking,  for  9  out  of  these  13  radicles  became  plainly, 
and  1  very  slightly,  curved  from  the  thick  paper 
towards  the  side  bearing  the  thin  paper.  In  two  of 
these  cases  the  apex  became  completely  hooked  after 
two  days;  in  four  cases  the  deflection  from  the  per- 
pendicular and  from  the  side  bearing  the  thick  paper, 
amounted  in  from  two  to  four  days  to  angles  of  90°, 
72°,  60°,  and  49°,  but  in  two  other  cases  to  only  18° 
and  15°.  It  should,  however,  be  stated  that  in  the 


CHAP.  III.  OF  THE  RADICLE   OF  ZEA.  177 

case  in  which  the  deflection  was  49°,  the  two  squares 
had  accidentally  come  into  contact  on  one  side  of  the 
apex,  and  thus  formed  a  lateral  gable ;  and  the  deflec- 
tion was  directed  in  part  from  this  gable  and  in  part 
from  the  thick  paper.  In  three  cases  alone  the  radicles 
were  not  affected  by  the  difference  in  thickness  of  the 
squares  of  paper  attached  to  their  tips,  and  conse- 
quently did  not  bend  away  from  the  side  bearing  the 
stiffer  paper. 

Zea  mays :  Sensitiveness  of  the  apex  of  the  Radicle  to 
contact. — A  large  number  of  trials  were  made  on  this 
plant,  as  it  was  the  only  monocotyledon  on  which  we 
experimented.  An  abstract  of  the  results  will  suffice. 
In  the  first  place,  22  germinating  seeds  were  pinned  to 
cork-lids  without  any  object  being  attached  to  their 
radicles,  some  being  exposed  to  a  temperature  of  65°-- 
66°  F.,  and  others  to  between  74°  and  79° ;  and  none  of 
them  became  curved,  though  some  were  a  little  inclined 
to  one  side.  A  few  were  selected,  which  from  having 
germinated  on  sand  were  crooked,  but  when  suspended 
in  damp  air  the  terminal  part  grew  straight  down- 
wards. This  fact  having  been  ascertained,  little  squares 
of  the  card -like  paper  were  affixed  with  shellac,  on 
several  occasions,  to  the  tips  of  68  radicles.  Of  these 
the  terminal  growing  part  of  39  became  within  24  h. 
conspicuously  curved  away  from  the  attached  squares 
and  from  the  perpendicular ;  13  out  of  the  39  forming 
hooks  with  their  points  directed  towards  the  zenith, 
and  8  forming  loops.  Moreover,  7  other  radicles  out 
of  the  68,  were  slightly  and  two  doubtfully  deflected 
from  the  cards.  There  remain  20  which  were  not 
affected ;  but  10  of  these  ought  not  to  be  counted ; 
for  one  was  diseased,  two  had  their  tips  quite  sur- 
rounded by  shellac,  and  the  squares  on  7  had  slipped 
so  as  to  stand  parallel  to  the  apex,  instead  of  obliquely 


178  SENSITIVENESS  OF  THE  APEX          CHAP.  III. 

on  it.  There  were  therefore  only  10  out  of  the  68 
which  certainly  were  not  acted  on.  Some  of  the 
radicles  which  were  experimented  on  were  young  and 
short,  most  of  them  of  moderate  length,  and  two  or 
three  exceeded  three  inches  in  length.  The  curva- 
ture in  the  above  cases  occurred  within  24  h.,  but  it 
was  often  conspicuous  within  a  much  shorter  period. 
For  instance,  the  terminal  growing  part  of  one  radicle 
was  bent  upwards  into  a  rectangle  in  8  h.  15  m.,  and 
of  another  in  9  h.  On  one  occasion  a  hook  was 
formed  in  9  h.  Six  of  the  radicles  in  a  jar  containing 
nine  seeds,  which  stood  on  a  sand-bath,  raised  to 
a  temperature  varying  from  76°  to  82°  F.,  became 
hooked,  and  a  seventh  formed  a  complete  loop,  when 
first  looked  at  after  15  hours. 

The  accompanying  figures  of  four  germinating  seeds 
(Fig.  69)  show,  firstly,  a  radicle  (A)  the  apex  of  which 
has  become  so  much  bent  away  from  the  attached 
square  as  to  form  a  hook.  Secondly  (B),  a  hook 
converted  through  the  continued  irritation  of  the 
card,  aided  perhaps  by  geotropism,  into  an  almost 
complete  circle  or  loop.  The  tip  in  the  act  of  forming 
a  loop  generally  rubs  against  the  upper  part  of  the 
radicle,  and  pushes  off  the  attached  square ;  the  loop 
then  contracts  or  closes,  but  never  disappears ;  and 
the  apex  afterwards  grows  vertically  downwards,  being 
no  longer  irritated  by  any  attached  object.  This 
frequently  occurred,  and  is  represented  at  C.  The 
jar  above  mentioned  with  the  six  hooked  radicles  and 
another  jar  were  kept  for  two  additional  days,  for  the 
sake  of  observing  how  the  hooks  would  be  modified. 
Most  of  them  became  converted  into  simple  loops, 
like  that  figured  at  C ;  but  in  one  case  the  apex  did 
not  rub  against  the  upper  part  of  the  radicle  and  thus 
remove  the  card;  and  it  consequently  made,  owing 


CHAP.  III.  OF  THE  EADICLE  OF  ZEA.  179 

to  the  continued  irritation  from  the  card,  two  complete 
loops,  that  is,  a  helix  of  two  spires ;  which  afterwards 
became  pressed  closely  together.  Then  geotropism 
prevailed  and  caused  the  apex  to  grow  perpendicularly 
downwards.  In  another  case,  shown  at  (D),  the  apex 

Fig.  69 


.0  D. 

Zea  mays :  radicles  excited  to  bend  away  from  the  little  squares  of  card 
attached  to  one  side  of  their  tips. 

in  making  a  second  turn  or  spire,  passed  through  the 
first  loop,  which  was  at  first  widely  open,  and  in 
doing  so  knocked  off  the  card ;  it  then  grew  perpen- 
dicularly downwards,  and  thus  tied  itself  into  a  knot, 
which  soon  became  tight ! 

Secondary  Radicles  of  Zea. — A  short  time  after  the 
first  radicle  has  appeared,  others  protrude  from  the 


180  SENSITIVENESS   OF   THE  APEX          CHAP.  in. 

seed,  but  not  laterally  from  the  primary  one.  Ten  of 
these  secondary  radicles,  which  were  directed  obliquely 
downwards,  were  experimented  on  with  very  small 
squares  of  card  attached  with  shellac  to  the  lower 
sides  of  their  tips.  If  therefore  the  squares  acted,  the 
radicles  would  bend  upwards  in  opposition  to  gravity. 
The  jar  stood  (protected  from  light)  on  a  sand-bath, 
which  varied  between  76°  and  82°  F.  After  only 
5  h.  one  appeared  to  be  a  little  deflected  from  the 
square,  and  after  20  h.  formed  a  loop.  Four  others 
were  considerably  curved  from  the  squares  after  20  h., 
and  three  of  them  became  hooked,  with  their  tips 
pointing  to  the  zenith, — one  after  29  h.  and  the 
two  others  after  44  h.  By  this  latter  time  a  sixth 
radicle  had  become  bent  at  a  right  angle  from  the  side 
bearing  the  square.  Thus  altogether  six  out  of  the 
ten  secondary  radicles  were  acted  on,  four  not  being 
affected.  There  can,  therefore,  be  no  doubt  that  the 
tips  of  these  secondary  radicles  are  sensitive  to  slight 
contact,  and  that  when  thus  excited  they  cause  the 
upper  part  to  bend  from  the  touching  object;  but 
generally,  as  it  appears,  not  in  so  short  a  time  as  in 
the  case  of  the  first-formed  radicle. 

SENSITIVENESS  OF  THE  TIP  OF  THE  RADICLE  TO 
MOIST  Am. 

Sachs  made  the  interesting  discovery,  a  few  years 
ago,  that  the  radicles  of  many  seedling  plants  bend 
towards  an  adjoining  damp  surface.*  We  shall  here 
endeavour  to  show  that  this  peculiar  form  of  sensitive- 
ness resides  in  their  tips.  The  movement  is  directly 
the  reverse  of  that  excited  by  the  irritants  hitherto 
considered,  which  cause  the  growing  part  of  the 

*  '  Arbeiten  des  Bot.  Institut.,  in  Wurzburg,'  vol.  i.  1872,  p.  209. 


CHAP.  III.       OF  THE   KADICLE   TO  MOIST  AIR.  181 

radicle  to  bend  away  from  the  source  of  irritation. 
In  our  experiments  we  followed  Sachs'  plan,  and  sieves 
with  seeds  germinating  in  damp  sawdust  were  sus- 
pended so  that  the  bottom  was  generally  inclined  at 
40°  with  the  horizon.  If  the  radicles  had  been  acted 
on  solely  by  geotropism,  they  would  have  grown  out 
of  the  bottom  of  the  sieve  perpendicularly  down- 
wards; but  as  they  were  attracted  by  the  adjoining 
damp  surface  they  bent  towards  it  and  were  deflected 
50°  from  the  perpendicular.  For  the  sake  of  ascertain- 
ing whether  the  tip  or  the  whole  growing  part  of  the 
radicle  was  sensitive  to  the  moist  air,  a  length  of  from 
1  to  2  mm.  was  coated  in  a  certain  number  of  cases 
with  a  mixture  of  olive-oil  and  lamp-black.  This 
mixture  was  made  in  order  to  give  consistence  to  the 
oil,  so  that  a  thick  layer  could  be  applied,  which 
would  exclude,  at  least  to  a  large  extent,  the  moist  air, 
and  would  be  easily  visible.  A  greater  number  of 
experiments  than  those  which  were  actually  tried 
would  have  been  necessary,  had  not  it  been  clearly 
established  that  the  tip  of  the  radicle  is  the  part  which 
is  sensitive  to  various  other  irritants. 

Phaseolus  muttiflorus. — Twenty-nine  radicles,  to  which  no- 
thing had  been  done,  growing  out  of  a  sieve,  were  observed 
at  the  same  time  with  those  which  had  their  tips  greased, 
and  for  an  equal  length  of  time.  Of  the  29,  24  curved  them- 
selves so  as  to  come  into  close  contact  with  the  bottom  of  the 
sieve.  The  place  of  chief  curvature  was  generally  at  a  distance 
of  5  or  6  mm.  from  the  apex.  Eight  radicles  had  their  tips 
greased  for  a  length  of  2  mm.,  and  two  others  for  a  length  of 
U  mm. ;  they  were  kept  at  a  temperature  of  15°-16°  C.  After 
intervals  of  from  19  h.  to  24  h.  all  were  still  vertically  or 
almost  vertically  dependent,  for  some  of  them  had  moved 
towards  the  adjoining  damp  surface  by  about  10°.  They  had 
therefore  not  been  acted  on,  or  only  slightly  acted  on,  by  the 
damper  air  on  one  side,  although  the  whole  upper  part  was 
freely  exposed.  After  48  h.  three  of  these  radicles  became 


182  SENSITIVENESS  OF   THE  APEX         CHAP.  ill. 

considerably  curved  towards  the  sieve  ;  and  the  absence  of  curva- 
ture- in  some  of  the  others  might  perhaps  be  accounted  for  by 
their  not  having  grown  very  well.  But  it  should  be  observed 
that  during  the  first  19  h.  to  24  h.  all  grew  well ;  two  of  them 
having  increased  2  and  3  mm.  in  length  in  11  h. ;  five  others 
increased  5  to  8  mm.  in  19  h. ;  and  two,  which  had  been  at  first 
4  and  6  mm.  in  length,  increased  in  24  h.  to  15  and  20  mm. 

The  tips  of  10  radicles,  which  likewise  grew  well,  were  coated 
with  the  grease  for  a  length  of  only  1  mm.,  and  now  the  result 
was  somewhat  different ;  for  of  these  4  curved  themselves  to 
the  sieve  in  from  21  h.  to  24  h.,  whilst  6  did  not  do  so. 
Five  of  the  latter  were  observed  for  an  additional  day,  and  now 
all  excepting  one  became  curved  to  the  sieve. 

The  tips  of  5  radicles  were  cauterised  with  nitrate  of  silver, 
and  about  1  mm.  in  length  was  thus  destroyed.  They  were 
observed  for  periods  varying  between  11  h.  and  24  h.,  and  wore 
found  to  have  grown  well.  One  of  them  had  curved  until  it 
came  into  contact  with  the  sieve ;  another  was  curving  towards 
it ;  whilst  the  remaining  three  were  still  vertically  dependent. 
Of  7  not  cauterised  radicles  observed  at  the  same  time,  all  had 
come  into  contact  with  the  sieve. 

The  tips  of  11  radicles  were  protected  by  moistened  gold- 
beaters' skin,  which  adheres  closely,  for  a  length  varying  from 
H  to  2|  mm.  After  22  h.  to  24  h.,  6  of  these  radicles  were 
clearly  bent  towards  or  had  come  into  contact  with  the  sieve ; 
2  were  slightly  curved  in  this  direction,  and  3  not  at  all.  All 
had  grown  well.  Of  14  control  specimens  observed  at  the  same 
time,  all  excepting  one  had  closely  approached  the  sieve.  It 
appears  from  these  cases  that  a  cap  of  goldbeaters'  skin  checks, 
though  only  to  a  slight  degree,  the  bending  of  the  radicles  to 
an  adjoining  damp  surface.  Whether  an  extremely  thin  sheet 
of  this  substance  when  moistened  allows  moisture  from  the  air 
to  pass  through  it,  we  do  not  know.  One  case  indicated  that 
the  caps  were  sometimes  more  efficient  than  appears  from  the 
above  results;  for  a  radicle,  which  after  23  h.  had  only 
slightly  approached  the  sieve,  had  its  cap  (1^  mm.  in  length) 
removed,  and  during  the  next  15 2  h.  it  curved  itself  abruptly 
towards  the  source  of  moisture,  the  chief  seat  of  curvature 
being  at  a  distance  of  2  to  3  mm.  from  the  apex. 

Vicia  faba. — The  tips  of  13  radicles  were  coated  with  the 
grease  for  a  length  of  2  mm. ;  and  it  should  be  remembered 
that  with  these  radicles  the  seat  of  chief  curvature  is  about 


CHAI>.  III.       OF   THE   RADICLE   TO  MOIST  Alii.  183 

4  or  5  mm.  from  the  apex.     Four  of  them  were  examined  after 
22  h.,  three  after  26  h.,  and  six  after  36  h.,  and  none  had 
been  attracted  towards  the  damp  lower  surface  of  the  sieve. 
In  another  trial  7  radicles  were  similarly  treated,  and  5  of  them 
still  pointed  perpendicularly  downwards  after   11   h.,  whilst 

2  were  a  little  curved  towards  the  sieve ;  by  an  accident  they 
were  not  subsequently  observed.     In   both  these  trials  the 
radicles  grew  well ;  7  of  them,  which  were  at  first  from  4  to 
11  mm.  in  length,  were  after  11  h.   between  7  and  16  mm. ; 

3  which  were  at  first  from  6  to  8  mm.  after  26  h.  were  11*5 
to  18  mm.  in  length ;  and  lastly,  4  radicles  which  were  at  first 

5  to  8  mm.  .after  46  h.  were   18  to  23  mm.  in  length.    The 
control  or  ungreased  radicles  were  not    invariably  attracted 
towards  the  bottom  of  the  sieve.     But  on  one  occasion  12  out  of 
13,  which  were  observed  for  periods  between  22  h.  and  36  h., 
were  thus  attracted.     On  two  other  occasions  taken  together, 
38  out  of  40  were  similarly  attracted.     On  another  occasion 
only  7  out  of  14  behaved  in  this  manner,  but  after  two  more 
days  the  proportion  of  the  curved  increased  to  17  out  of  23. 
On  a  last  occasion  only  11  out  of  20  were  thus  attracted.    If 
we  add  up  these  numbers,  we  find  that  78  out  of  96  of  the 
control  specimens  curved  themselves  towards  the  bottom  of  the 
sieve.    Of  the  specimens  with  greased  tips,  2  alone  out  of  the 
20  (but  7  of  these  were  not  observed  for  a  sufficiently  long 
time)  thus  curved  themselves.    We  can,  therefore,  hardly  doubt 
that  the  tip  for  a  length  of  2  mm.  is  the  part  which  is  sensitive 
to  a  moist  atmosphere,  and  causes  the  upper  part  to  bend 
towards  its  source. 

The  tips  of  15  radicles  were  cauterised  with  nitrate  of  silver, 
and  they  grew  as  well  as  those  above  described  with  greased 
tips.  After  an  interval  of  24  h.,  9  of  them  were  not  at  all 
curved  towards  the  bottom  of  the  sieve ;  2  were  curved  towards 
it  at  angles  of  20°  and  12°  from  their  former  vertical  position, 
and  4  had  come  into  close  contact  with  it.  Thus  the  destruc- 
tion of  the  tip  for  a  length  of  about  1  mm.  prevented  the  curva- 
ture of  the  greater  number  of  these  radicles  to  the  adjoining 
damp  surface.  Of  24  control  specimens,  23  were  bent  to  the 
sieve,  and  on  a  second  occasion  15  out  of  16  were  similarly 
curved  in  a  greater  or  less  degree.  These  control  trials  are 
included  in  those  given  in  the  foregoing  paragraph. 

Averta  sativa. — The  tips  of  13  radicles,  which  projected 
between  2  and  4  mm.  from  the  bottom  of  the  sieve,  many  of 
9 


184  SENSITIVENESS   OF   THE  APEX         CHAP.  III. 

them  not  quite  perpendicularly  downwards,  were  coated  with 
the  black  grease  for  a  length  of  from  1  to  lg  mm.  The  sieves 
were  inclined  at  30°  with  the  horizon.  The  greater  number  of 
these  radicles  were  examined  after  22  h.,  and  a  few  after  25  h., 
and  within  these  intervals  they  had  grown  so  quickly  as  to  have 
nearly  doubled  their  lengths.  With  the  ungreased  radicles  the 
chief  seat  of  curvature  is  at  a  distance  of  not  less  than  between 
3 '5  and  5'5  nun.,  and  not  more  than  between  7  and  10  mm.  from 
the  apex.  Out  of  the  13  radicles  with  greased  tips,  4  had  not 
moved  at  all  towards  the  sieve ;  6  were  deflected  towards  it  and 
from  the  perpendicular  by  angles  varying  between  10°  and  35°  ; 
and  3  had  come  into  close  contact  with  it.  It  appears,  therefore, 
at  first  sight  that  greasing  the  tips  of  these  radicles  had  checked 
but  little  their  bending  to  the  adjoining  damp  surface.  But  the 
inspection  of  the  sieves  on  two  occasions  produced  a  widely 
different  impression  on  the  mind;  for  it  was  impossible  to 
behold  the  radicles  with  the  black  greased  tips  projecting  from 
the  bottom,  and  all  those  with  ungreased  tips,  at  least  40  to  50 
in  number,  clinging  closely  to  it,  and  feel  any  doubt  that  the 
greasing  had  produced  a  great  effect.  On  close  examination 
only  a  single  ungreased  radicle  could  be  found  which  had  not 
become  curved  towards  the  sieve.  It  is  probable  that  if  the 
tips  had  been  protected  by  grease  for  a  length  of  2  mm.  instead 
of  from  1  to  Is  mm ,  they  would  not  have  been  affected  by  the 
moist  air  and  none  would  have  become  curved. 

Triticum  vulgare. — Analogous  trials  were  made  on  8  radicles 
of  the  common  wheat ;  and  greasing  their  tips  produced  much 
less  effect  than  in  the  case  of  the  oats.  After  22  h.,  5  of  them 
had  come  into  contact  with  the  bottom  of  the  sieve ;  2  had 
moved  towards  it  10°  and  15°,  and  one  alone  remained  perpen- 
dicular. Not  one  of  the  very  numerous  ungreased  radicles 
failed  to  come  into  close  contact  with  the  sieve.  These  trials 
were  made  on  Nov.  28th,  when  the  temperature  was  only  4°'8  C. 
at  10  A.M.  We  should  hardly  have  thought  this  case  worth 
notice,  had  it  not  been  for  the  following  circumstance.  In  the 
beginning  of  October,  when  the  temperature  was  considerably 
higher,  viz.,  12°  to  13°  C.,  we  found  that  only  a  few  of  the 
ungreased  radicles  became  bent  towards  the  sieve;  and  this 
indicates  that  sensitiveness  to  moisture  in  the  air  is  increased 
by  a  low  temperature,  as  we  have  seen  with  the  radicles  of 
Vicia  faba  relatively  to  objects  attached  to  their  tips.  But  in 
the  present  instance  it  is  possible  that  a  difference  in  the  dryness 


CHAP.  III.        OF   THE   KADICLE   TO   MOIST  AIR.  185 

of  the  air  may  have  caused  the  difference  in  the  results  at  the 
two  periods. 

Finally,  the  facts  just  given  with  respect  to  Phaseolus 
multiflorus,  Vicia  faba,  and  Avena  sativa  show,  as  it 
seems  to  us,  that  a  layer  of  grease  spread  for  a  length 
of  1J  to  2  mm.  over  the  tip  of  the  radicle,  or  the 
destruction  of  the  tip  by  caustic,  greatly  lessens  or 
quite  annuls  in  the  upper  and  exposed  part  the  power 
of  bending  towards  a  neighbouring  source  of  moisture. 
We  should  bear  in  mind  that  the  part  which  bends 
most,  lies  at  some  little  distance  above  the  greased  or 
cauterised  tip  ;  and  that  the  rapid  growth  of  this  part, 
proves  that  it  has  not  been  injured  by  the  tips  having 
been  thus  treated.  In  those  cases  in  which  the  radicles 
with  greased  tips  became  curved,  it  is  possible  that  the 
layer  of  grease  was  not  sufficiently  thick  wholly  to  ex- 
clude moisture,  or  that  a  sufficient  length  was  not  thus 
protected,  or,  in  the  case  of  the  caustic,  not  destroyed. 
When  radicles  with  greased  tips  are  left  to  grow  for 
several  days  in  damp  air,  the  grease  is  drawn  out  into 
the  finest  reticulated  threads  and  dots,  with  narrow 
portions  of  the  surface  left  clean.  Such  portions 
would,  it  is  probable,  be  able  to  absorb  moisture,  and 
thus  we  can  account  for  several  of  the  radicles  with 
greased  tips  having  become  curved  towards  the  sieve 
after  an  interval  of  one  or  two  days.  On  the  whole, 
we  may  infer  that  sensitiveness  to  a  difference  in  the 
amount  of  moisture  in  the  air  on  the  two  sides  of  a 
radicle  resides  in  the  tip,  which  transmits  some  influ- 
ence to  the  upper  part,  causing  it  to  bend  towards  the 
source  of  moisture.  Consequently,  the  movement  is 
the  reverse  of  that  caused  by  objects  attached  to  one 
side  of  the  tip,  or  by  a  thin  slice  being  cut  off,  or  by 
being  slightly  cauterised.  In  a  future  chapter  it 
will  be  shown  that  sensitiveness  to  the  attraction  of 


186  THE   EFFECT   OF   KILLING   OR         CHAP.  III. 

gravity  likewise  resides  in  the  tip;  so  that  it  is  the 
tip  which  excites  the  adjoining  parts  of  a  horizontally 
extended  radicle  to  bend  towards  the  centre  of  the 
earth. 

SECONDARY  EADICLES  BECOMING  VERTICALLY  GEO- 
TROPIC  BY  THE  DESTRUCTION  OR  INJURY  OF  THE 
TERMINAL  PART  OF  THE  PRIMARY  RADICLE. 

Sachs  has  shown  that  the  lateral  or  secondary 
radicles  of  the  bean,  and  probably  of  other  plants,  are 
acted  on  by  geotropism  in  so  peculiar  a  manner,  that 
they  grow  out  horizontally  or  a  little  inclined  down- 
wards ;  and  he  has  further  shown*  the  interesting  fact, 
that  if  the  end  of  the  primary  radicle  be  cut  off,  one 
of  the  nearest  secondary  radicles  changes  its  nature 
and  grows  perpendicularly  downwards,  thus  replacing 
the  primary  radicle.  We  repeated  this  experiment, 
and  planted  beans  with  amputated  radicles  in  friable 
peat,  and  saw  the  result  described  by  Sachs ;  but 
generally  two  or  three  of  the  secondary  radicles  grew 
perpendicularly  downwards.  We  also  modified  the 
experiment,  by  pinching  young  radicles  a  little  way 
above  their  tips,  between  the  arms  of  a  U-shaped 
piece  of  thick  leaden  wire.  The  part  pinched  was 
thus  flattened,  and  was  afterwards  prevented  from 
growing  thicker.  Five  radicles  had  their  ends  cut 
off,  and  served  as  controls  or  standards.  Eight  were 
pinched ;  of  these  2  were  pinched  too  severely  and 
their  ends  died  and  dropped  off ;  2  were  not  pinched 
enough  and  were  not  sensibly  affected  ;  the  remaining 
4  were  pinched  sufficiently  to  check  the  growth  of 
the  terminal  part,  but  did  not  appear  otherwise  injured. 
When  the  U-shaped  wires  were  removed,  after  an 


*  « Arbeiten  Bot.  Institut.,  Wurzburg,'  Heft  iv.  1874,  p.  622. 


CHAP.  III.     INJURING  THE  PRIMARY  KADICLE.  187 

interval  of  15  days,  the  part  beneath  the  wire  was 
found  to  be  very  thin  and  easily  broken,  whilst  the 
part  above  was  thickened.  Now  in  these  four  cases, 
one  or  more  of  the  secondary  radicles,  arising  from 
the  thickened  part  just  above  the  wire,  had  grown 
perpendicularly  downwards.  In  the  best  case  the 
primary  radicle  (the  part  below  the  wire  being  1J  inch 
in  length)  was  somewhat  distorted,  and  was  not  half 
as  long  as  three  adjoining  secondary  radicles,  which 
had  grown  vertically,  or  almost  vertically,  downwards. 
Some  of  these  secondary  radicles  adhered  together  or 
had  become  confluent.  We  learn  from  these  four  cases 
that  it  is  not  necessary,  in  order  that  a  secondary 
radicle  should  assume  the  nature  of  a  primary  one, 
that  the  latter  should  be  actually  amputated  ;  it  is 
sufficient  that  the  flow  of  sap  into  it  should  be 
checked,  and  consequently  should  be  directed  into  the 
adjoining  secondary  radicles ;  for  this  seems  to  be 
the  most  obvious  result  of  the  primary  radicle  being 
pinched  between  the  arms  of  a  U-shaped  wire. 

This  change  in  the  nature  of  secondary  radicles  is 
clearly  analogous,  as  Sachs  has  remarked,  to  that 
which  occurs  with  the  shoots  of  trees,  when  the  leading 
one  is  destroyed  and  is  afterwards  replaced  by  one  or 
more  of  the  lateral  shoots  ;  for  these  now  grow  upright 
instead  of  sub-horizontally.  But  in  this  latter  case 
the  lateral  shoots  are  rendered  apogeotropic,  whereas 
with  radicles  the  lateral  ones  are  rendered  geotropic. 
We  are  naturally  led  to  suspect  that  the  same  cause 
acts  with  shoots  as  with  roots,  namely,  an  increased  flow 
of  sap  into  the  lateral  ones.  We  made  some  trials  with 
Abies  communis  and  pectinata,  by  pinching  with  wire 
the  leading  and  all  the  lateral  shoots  excepting  one. 
But  we  believe  that  they  were  too  old  when  experi- 
mented on  ;  and  some  were  pinched  too  severely,  and 


188  THE  EFFECT  OF  KILLING  OR          CHAP.  III. 

some  not  enough.  Only  one  case  succeeded,  namely, 
with  the  spruce-fir.  The  leading  shoot  was  not  killed, 
but  its  growth  was  checked ;  at  its  base  there  were 
three  lateral  shoots  in  a  whorl,  two  of  which  were 
pinched,  one  being  thus  killed ;  the  third  was  left 
untouched.  These  lateral  shoots,  when  operated  on 
(July  14th)  stood  at  an  angle  of  8°  above  the  horizon  ; 
by  Sept.  8th  the  unpinched  one  had  risen  35° ;  by 
Oct.  4th  it  had  risen  46°,  and  by  Jan.  26th  48°,  and 
it  had  now  become  a  little  curved  inwards.  Part 
of  this  rise  of  48°  may  be  attributed  to  ordinary 
growth,  for  the  pinched  shoot  rose  12°  within  the  same 
period.  It  thus  follows  that  the  unpinched  shoot 
stood,  on  Jan.  26th,  56°  above  the  horizon,  or  34° 
from  the  vertical;  and  it  was  thus  obviously  almost 
ready  to  replace  the  slowly  growing,  pinched,  lead- 
ing shoot.  Nevertheless,  we  feel  some  doubt  about 
this  experiment,  for  we  have  since  observed  with 
spruce-firs  growing  rather  unhealthily,  that  the  lateral 
shoots  near  the  summit  sometimes  become  highly 
inclined,  whilst  the  leading  shoot  remains  apparently 
sound. 

A  widely  different  agency  not  rarely  causes  shoots 
which  naturally  would  have  grown  out  horizontally  to 
grow  up  vertically.  The  lateral  branches  of  the  Silver 
Fir  (A.  pectinata)  are  often  affected  by  a  fungus, 
JEcidium  elatinum,  which  causes  the  branch  to  enlarge 
into  an  oval  knob  formed  of  hard  wood,  in  one  of 
which  we  counted  24  rings  of  growth.  According  to 
De  Bary,*  when  the  mycelium  penetrates  a  bud  be- 
ginning to  elongate,  the  shoot  developed  from  it 
grows  vertically  upwards.  Such  upright  shoots  after- 


*  See  his  valuable  article  in      are   called  in   German   "  Hexen- 
'  Bot.  Zeitunor,'  1867,  p.  257,  on       besen,"  or  "  witch-brooms." 
these  monstrous  growths,,  which 


CHAP.  III.      INJUKING  THE   PRIMARY   RADICLE.  189 

wards  produce  lateral  and  horizontal  branches;  and 
they  then  present  a  curious  appearance,  as  if  a  young 
fir-tree  had  grown  out  of  a  ball  of  clay  surrounding 
the  branch.  These  upright  shoots  have  manifestly 
changed  their  nature  and  become  apogeotropic ;  for  if 
they  had  not  been  affected  by  the  .ZEcidium,  they 
would  have  grown  out  horizontally  like  all  the  other 
twigs  on  the  same  branches.  This  change  can  hardly 
be  due  to  an  increased  flow  of  sap  into  the  part ;  but 
the  presence  of  the  mycelium  will  have  greatly  dis- 
turbed its  natural  constitution. 

According  to  Mr.  Meehan,*  the  stems  of  three 
species  of  Euphorbia  and  of  Portulaca  oleraeea  are 
"  normally  prostrate  or  procumbent ;"  but  when  they 
are  attacked  by  an  ^Ecidium,  they  "  assume  an  erect 
habit."  Dr.  Stahl  informs  us  that  he  knows  of  several 
analogous  cases ;  and  these  seem  to  be  closely  related 
to  that  of  the  Abies.  The  rhizomes  of  Sparganium 
ramosum  grow  out  horizontally  in  the  soil  to  a  con- 
siderable length,  or  are  diageotropic ;  but  F.  Elfving 
found  that  when  they  were  cultivated  in  water 
their  tips  turned  upwards,  and  they  became  apogeo- 
tropic. The  same  result  followed  when  the  stem  of  the 
plant  was  bent  until  it  cracked  or  was  merely  much 
bowed.f 

No  explanation  has  hitherto  been  attempted  of  such 
cases  as  the  foregoing, — namely,  of  secondary  radicles 
growing  vertically  downwards,  and  of  lateral  shoots 
growing  vertically  upwards,  after  the  amputation  of 


*  '  Proc.  Acad.  Nat.  Sc.  Phila-  viously  observed  (<  Flora,'  1878, 

delphia,'  June  16th,  1874,  and  p.  324)  that  the  underground 

July  23rd,  1875.  shoots  of  Triticum  repens  bend 

t  See  F.  Elfving's  interesting  vertically  up  when  the  parts  above 

paper  in  '  Arbeiten  Bot.  Institut.,  ground  are  removed,  and  when 

in  Wiirzburg,'  vol.  ii.  1880,  p.  489.  the  rhizomes  are  kept  portly  im- 

Carl  Kraus  (Triesdorf)  had  pie-  mersid  in  water. 


190    EFFECT  OF  KILLING  PEIMARY  RADICLE.   CHAP.  III. 

the  primary  radicle  or  of  the  leading  shoot.  The 
following  considerations  give  us,  as  we  believe,  the 
clue.  Firstly,  any  cause  which  disturbs  the  con- 
stitution *  is  apt  to  induce  reversion ;  such  as  the 
crossing  of  two  distinct  races,  or  a  change  of  con- 
ditions, as  when  domestic  animals  become  feral. 
But  the  case  which  most  concerns  us,  is  the  frequent 
appearance  of  peloric  flowers  on  the  summit  of  a  stem, 
or  in  the  centre  of  the  inflorescence, — parts  which,  it  is 
believed,  receive  the  most  sap ;  for  when  an  irregular 
flower  becomes  perfectly  regular  or  peloric,  this  may 
be  attributed,  at  least  partly,  to  reversion  to  a  primi- 
tive and  normal  type.  Even  the  position  of  a  seed  at 
the  end  of  the  capsule  sometimes  gives  to  the  seedling 
developed  from  it  a  tendency  to  revert.  Secondly, 
reversions  often  occur  by  means  of  buds,  independently 
of  reproduction  by  seed  ;  so  that  a  bud  may  revert  to 
the  character  of  a  former  state  many  bud-generations 
ago.  In  the  case  of  animals,  reversions  may  occur  in 
the  individual  with  advancing  age.  Thirdly  and 
lastly,  radicles  when  they  first  protrude  from  the  seed 
are  always  geotropic,  and  plumules  or  shoots  almost 
always  apogeotropic.  If  then  any  cause,  such  as  an 
increased  flow  of  sap  or  the  presence  of  mycelium, 
disturbs  the  constitution  of  a  lateral  shoot  or  of  a 
secondary  radicle,  it  is  apt  to  revert  to  its  primordial 
state ;  and  it  becomes  either  apogeotropic  or  geotropic, 
as  the  case  may  be,  and  consequently  grows  either 
vertically  upwards  or  downwards.  It  is  indeed  pos- 


*  The  facts  on  which  the  fol-  xiv.      On   piloric   flowers,   chap, 

lowing  conclusions   are  founded  xiii.  p.  32  ;  and  see  p.  337  on  their 

are  given   in  *  The  Variation  of  position    on    the     plant.      With 

Animals  and  Plants  under  Domes-  respect  to  seeds,  p.  340.     On  re- 

tication,'  2nd  edit.  1875.     Ou  the  version  by  means  of  buds,  p.  438, 

causes  leading   to  reversion    see  chap.  xi.  vol.  i. 
chap.  xii.  vol.  ii.  and  p.  59,  chap. 


CHAP.  III.  SUMMARY  OF   CHAPTER.  191 

sible,  or  even  probable,  that  this  tendency  to  reversion 
may  have  been  increased,  as  it  is  manifestly  of  service 
to  the  plant. 

SUMMARY   OF    CHAPTER. 

A  part  or  organ  may  be  called  sensitive,  when  its 
irritatioii  excites  movement  in  an  adjoining  part.  Now 
it  has  been  sho\vn  in  this  chapter,  that  the  tip  of  the 
radicle  of  the  bean  is  in  this  sense  sensitive  to  the 
contact  of  any  small  object  attached  to  one  side  by 
shellac  or  gum-water ;  also  to  a  slight  touch  with  dry 
caustic,  and  to  a  thin  slice  cut  oft*  one  side.  The 
radicles  of  the  pea  were  tried  with  attached  objects 
and  caustic,  both  of  which  acted.  With  Phaseolus 
multiflorus  the  tip  was  hardly  sensitive  to  small  squares 
of  attached  card,  but  was  sensitive  to  caustic  and  to 
slicing.  The  radicles  of  Tropaeolum  were  highly  sen- 
sitive to  contact ;  and  so,  as  far  as  we  could  judge, 
were  those  of  Gossypium  herbaceum,  and  they  were 
certainly  sensitive  to  caustic.  The  tips  of  the  radicles 
of  Cucurbita  ovifera  were  likewise  highly  sensitive  to 
caustic,  though  only  moderately  so  to  contact.  Ea- 
plianus  sativus  offered  a  somewhat  doubtful  case. 
With  ^Esculus  the  tips  were  quite  indifferent  to 
bodies  attached  to  them,  though  sensitive  to  caustic. 
Those  of  Quercus  robur  and  Zea  mays  were  highly  sen- 
sitive to  contact,  as  were  the  radicles  of  the  latter 
to  caustic.  In  several  of  these  cases  the  difference  in 
sensitiveness  of  the  tip  to  contact  and  to  caustic  was, 
as  we  believe,  merely  apparent ;  for  with  Gossypium, 
Raphanus,  and  Cucurbita,  the  tip  was  so  fine  and 
flexible  that  it  was  very  difficult  to  attach  any  object 
to  one  of  its  sides.  With  the  radicles  of  ./Esculus, 
the  tips  were  not  at  all  sensitive  to  small  bodies 
attached  to  them ;  but  it  does  not  follow  from  this 


192  SUMMARY   OF  .CHAPTER.  CIJAP.  III. 

fact  that  they  would  not  have  been  sensitive  to  some- 
what greater  continued  pressure,  if  this  could  have 
been  applied. 

The  peculiar  form  of  sensitiveness  which  we  are 
here  considering,  is  confined  to  the  tip  of  the  radicle 
for  a  length  of  from  1  mm.  to  1  *  5  mm.  When  this 
part  is  irritated  by  contact  with  any  object,  by  caustic, 
or  by  a  thin  slice  being  cut  off,  the  upper  adjoining 
part  of  the  radicle,  for  a  length  of  from  6  or  7  to 
even  12  mm.,  is  excited  to  bend  away  from  the  side 
which  has  been  irritated.  Some  influence  must  there- 
fore be  transmitted  from  the  tip  along  the  radicle  for 
this  length.  The  curvature  thus  caused  is  generally 
symmetrical.  The  part  which  bends  most  apparently 
coincides  with  that  of  the  most  rapid  growth.  The 
tip  and  the  basal  part  grow  very  slowly  and  they 
bend  very  little. 

Considering  the  widely  separated  position  in  the 
vegetable  series  of  the  several  above-named  genera, 
we  may  conclude  that  the  tips  of  the  radicles  of  all,  or 
almost  all,  plants  are  similarly  sensitive,  and  transmit 
an  influence  causing  the  upper  part  to  bend.  With 
respect  to  the  tips  of  the  secondary  radicles,  those  of 
Vicia  faba,  Pisum  sativum,  and  Zea  mays  were  alone 
observed,  and  they  were  found  similarly  sensitive. 

In  order  that  these  movements  should  be  properly 
displayed,  it  appears  necessary  that  the  radicles 
should  grow  at  their  normal  rate.  If  subjected  to  a 
high  temperature  and  made  to  grow  rapidly,  the 
tips  seem  either  to  lose  their  sensitiveness,  or  the 
upper  part  to  lose  the  power  of  bending.  So  it 
appears  to  be  if  they  grow  very  slowly  from  not  being 
vigorous,  or  from  being  kept  at  too  low  a  temperature ; 
also  when  they  are  forced  to  germinate  in  the  middle 
of  the  winter. 


CHAP.  III.  SUMMARY   OF  CHAPTER.  193 

The  curvature  of  the  radicle  sometimes  occurs 
within  from  6  to  8  hours  after  the  tip  has  been  irritated, 
and  almost  always  within  24  h.,  excepting  in  the 
case  of  the  massive  radicles  of  ^Esculus.  The  curva- 
ture often  amounts  to  a  rectangle, — that  is,  the  ter- 
minal part  bends  upwrards  until  the  tip,  which  is  but 
little  curved,  projects  almost  horizontally.  Occa- 
sionally the  tip,  from  the  continued  irritation  of  the 
attached  object,  continues  to  bend  up  until  it  forms  a 
hook  with  the  point  directed  towards  the  zenith,  or 
a  loop,  or  even  a  spire.  After  a  time  the  radicle 
apparently  becomes  accustomed  to  the  irritation,  as 
occurs  in  the  case  of  tendrils,  for  it  again  grows  down- 
wards, although  the  bit  of  card  or  other  object  may 
remain  attached  to  the  tip. 

It  is  evident  that  a  small  object  attached  to  the  free 
point  of  a  vertically  suspended  radicle  can  offer  no 
mechanical  resistance  to  its  growth  as  a  whole,  for  the 
object  is  carried  downwards  as  the  radicle  elongates, 
or  upwards  as  the  radicle  curves  upwards.  Nor  can 
the  growth  of  the  tip  itself  be  mechanically  checked 
by  an  object  attached  to  it  by  gum-water,  which 
remains  all  the  time  perfectly  soft.  The.  weight  of 
the  object,  though  quite  insignificant,  is  opposed 
to  the  upward  curvature.  We  may  therefore  conclude 
that  it  is  the  irritation  due  to  contact  which  excites 
the  movement.  The  contact,  however,  must  be  pro- 
longed, for  the  tips  of  15  radicles  were  rubbed  for  a 
short  time,  and  this  did  not  cause  them  to  bend.  Here 
then  we  have  a  case  of  specialised  sensibility,  like 
that  of  the  glands  of  Drosera ;  for  these  are  ex- 
quisitely sensitive  to  the  slightest  pressure  if  prolonged, 
but  not  to  two  or  three  rough  touches. 

When  the  tip  of  a  radicle  is  lightly  touched  on  one 
side  with  dry  nitrate  of  silver,  the  injury  caused  is 


191  SUMMARY   OF   CHAPTER.  CHAP.  III. 

very  slight,  and  the  adjoining  upper  part  bends  away 
from  the  cauterised  point,  with  more  certainty  in  most 
cases  than  from  an  object  attached  on  one  side.  Here 
it  obviously  is  not  the  mere  touch,  but  the  effect 
produced  by  the  caustic,  which  induces  the  tip  to 
transmit  some  influence  to  the  adjoining  part,  causing 
it  to  bend  away.  If  one  side  of  the  tip  is  badly 
injured  or  killed  by  the  caustic,  it  ceases  to  grow, 
whilst  the  opposite  side  continues  growing  ;  and  the 
result  is  that  the  tip  itself  bends  towards  the  injured 
side  and  often  becomes  completely  hooked ;  and  it  is 
remarkable  that  in  this  case  the  adjoining  upper  part 
does  not  bend.  The  stimulus  is  too  powerful  or  the 
shock  too  great  for  the  proper  influence  to  be  trans- 
mitted from  the  tip.  We  have  strictly  analogous  cases 
with  Drosera,  Dionrea  and  Pinguicula,  with  which 
plants  a  too  powerful  stimulus  does  not  excite  the 
tentacles  to  become  incurved,  or  the  lobes  to  close,  or 
the  margin  to  be  folded  inwards. 

With  respect  to  the  degree  of  sensitiveness  of  the 
apex  to  contact  under  favourable  conditions,  we  have 
seen  that  with  Vicia  faba  a  little  square  of  writing- 
paper  affixed  with  shellac  sufficed  to  cause  move- 
ment; as  did  on  one  occasion  a  square  of  merely 
damped  goldbeaters'  skin,  but  it  acted  very  slowly. 
Short  bits  of  moderately  thick  bristle  (of  which  mea- 
surements have  been  given)  affixed  with  gum-water 
acted  in  only  three  out  of  eleven  trials,  and  beads  of 
dried  shellac  under  2  jo^h  °f  a  grain  in  weight  acted 
only  twice  in  nine  cases ;  so  that  here  we  have 
nearly  reached  the  minimum  of  necessary  irrita- 
tion. The  apex,  therefore,  is  much  less  sensitive  to 
pressure  than  the  glands  of  Drosera,  for  these  are 
affected  by  far  thinner  objects  than  bits  of  bristle, 
and  by  a  very  much  less  weight  than 


CHAP.  III.  SUMMARY   OF   CHAPTER.  195 

But  the  most  interesting  evidence  of  the  delicate 
sensitiveness  of  the  tip  of  the  radicle,  was  afforded  by 
its  power  of  discriminating  between  equal-sized  squares 
of  card-like  and  very  thin  paper,  when  these  were 
attached  on  opposite  sides,  as  was  observed  with  the 
radicles  of  the  bean  and  oak. 

When  radicles  of  the  bean  are  extended  horizon- 
tally with  squares  of  card  attached  to  the  lower  sides  of 
their  tips,  the  irritation  thus  caused  was  always  con- 
quered by  geotropism,  which  then  acts  under  the  most 
favourable  conditions  at  right  angles  to  the  radicle. 
But  when  objects  were  attached  to  the  radicles  of  and 
of  the  above-named  genera,  suspended  vertically,  the 
irritation  conquered  geotropism,  which  latter  power 
at  first  acted  obliquely  on  the  radicle ;  so  that  the 
immediate  irritation  from  the  attached  object,  aided 
by  its  after-effects,  prevailed  and  caused  the  radicle 
to  bend  upwards,  until  sometimes  the  point  was 
directed  to  the  zenith.  We  must,  however,  assume 
that  the  after-effects  of  the  irritation  of  the  tip  by  an 
attached  object  come  into  play,  only  after  movement 
has  been  excited.  The  tips  of  the  radicles  of  the  pea 
seem  to  be  more  sensitive  to  contact  than  those  of  the 
bean,  for  when  they  were  extended  horizontally  with 
squares  of  card  adhering  to  their  lower  sides,  a  most 
curious  struggle  occasionally  arose,  sometimes  one 
and  sometimes  the  other  force  prevailing,  but  ulti- 
mately geotropism  was  always  victorious ;  neverthe- 
less, in  two  instances  the  terminal  part  became  so 
much  curved  upwards  that  loops  were  subsequently 
formed.  With  the  pea,  therefore,  the  irritation  from 
an  attached  object,  and  from  geotropism  when  acting 
at  right  angles  to  the  radicle,  are  nearly  balanced 
forces.  Closely  similar  results  were  observed  with  the 
horizontally  extended  radicles  of  Cucurbita  ovifera, 


196  SUMMARY   OF   CHAPTER.  CHAP.  III. 

when  their  tips  were  slightly  cauterised  on  the  lower 
side. 

Finally,  the  several  co-ordinated  movements  by 
which  radicles  are  enabled  to  perform  their  proper 
functions  are  ^admirably  perfect.  In  whatever  direc- 
tion the  primary  radicle  first  protrudes  from  the  seed, 
geotropism  guides  it  perpendicularly  downwards ;  and 
the  capacity  to  be  acted  on  by  the  attraction  of 
gravity  resides  in  the  tip.  But  Sachs  has  proved* 
that  the  secondary  radicles,  or  those  emitted  by  the 
primary  one,  are  acted  on  by  geotropism  in  such  a 
manner  that  they  tend  to  bend  only  obliquely  down- 
wards. If  they  had  been  acted  on  like  the  primary 
radicle,  all  the  radicles  would  have  penetrated  the 
ground  in  a  close  bundle.  We  have  seen  that  if 
the  end  of  the  primary  radicle  is  cut  off  or  in- 
jured, the  adjoining  secondary  radicles  become  geo- 
tropic  and  grow  vertically  downwards.  This  power 
must  often  be  of  great  service  to  the  plant,  when  the 
primary  radicle  has  been  destroyed  by  the  larvae  of 
insects,  burrowing  animals,  or  any  other  accident.  The 
tertiary  radicles,  or  those  emitted  by  the  secondary 
ones,  are  not  influenced,  at  least  in  the  case  of  the 
bean,  by  geotropism ;  so  they  grow  out  freely  in  all 
directions.  From  this  manner  of  growth  of  the  various 
kinds  of  radicles,  they  are  distributed,  together  with 
their  absorbent  hairs,  throughout  the  surrounding  soil, 
as  Sachs  has  remarked,  in  the  most  advantageous 
manner  ;  for  the  whole  soil  is  thus  closely  searched. 

Geotropism,  as  was  shown  in  the  last  chapter, 
excites  the  primary  radicle  to  bend  downwards  with 
very  little  force,  quite  insufficient  to  penetrate  the 
ground.  Such  penetration  is  effected  by  the  pointed 


*  l  Arbeiten  Hot.  Institut.,  Wiirzburg,'  Heft  iv.  1874,  pp.  605-631. 


CHAP.  III.  SUMMARY   OF   CHAPTER.  197 

apex  (protected  by  the  root-cap)  being  pressed  down 
by  the  longitudinal  expansion  or  growth  of  the  ter- 
minal rigid  portion,  aided  by  its  transverse  expan- 
sion, both  of  which  forces  act  powerfully.  It  is, 
however,  indispensable  that  the  seeds  should  be  at 
first  held  down  in  some  manner.  When  they  lie 
on  the  bare  surface  they  are  held  down  by  the  attach- 
ment of  the  root-hairs  to  any  adjoining  objects ;  and 
this  apparently  is  effected  by  the  conversion  of 
their  outer  surfaces  into  a  cement.  But  many  seeds 
get  covered  up  by  various  accidents,  or  they  fall  into 
crevices  or  holes.  With  some  seeds  their  own  weight 
suffices. 

The  circumnutating  movement  of  the  terminal  grow- 
ing part  both  of  the  primary  and  secondary  radicles 
is  so  feeble  that  it  can  aid  them  very  little  in  pene- 
trating the  ground,  excepting  when  the  superficial 
layer  is  very  soft  and  damp.  But  it  must  aid  them 
materially  w7hen  they  happen  to  break  obliquely  into 
cracks,  or  into  burrows  made  by  earth-worms  or  larva?. 
This  movement,  moreover,  combined  with  the  sen- 
sitiveness of  the  tip  to  contact,  can  hardly  fail  to  be 
of  the  highest  importance;  for  as  the  tip  is  always 
endeavouring  to  bend  to  all  sides  it  will  press  on  all 
sides,  and  will  thus  be  able  to  discriminate  between 
the  harder  and  softer  adjoining  surfaces,  in  the  same 
manner  as  it  discriminated  between  the  attached 
squares  of  card-like  and  thin  paper.  Consequently  it 
will  tend  to  bend  from  the  harder  soil,  and  will  thus 
follow  the  lines  of  least  resistance.  So  it  will  be  if  it 
meets  with  a  stone  or  the  root  of  another  plant  in  the 
soil,  as  must  incessantly  occur.  If  the  tip  were  not 
sensitive,  and  if  it  did  not  excite  the  upper  part  of  the 
root  to  bend  away,  whenever  it  encountered  at  right 
angles  some  obstacle  in  the  ground,  it  would  be  liable 


198  SUMMARY   OF  CHAPTER.  CHAP.  III. 

to  be  doubled  up  into  a  contorted  mass.  But  we  have 
seen  with,  radicles  growing  down  inclined  plates  of 
glass,  that  as  soon  as  the  tip  merely  touched  a  slip  of 
wood  cemented  across  the  plate,  the  whole  terminal 
growing  part  curved  away,  so  that  the  tip  soon  stood 
at  right  angles  to  its  former  direction;  and  thus  it 
would  be  with  an  obstacle  encountered  in  the  ground, 
as  far  as  the  pressure  of  the  surrounding  soil  would 
permit.  We  can  also  understand  why  thick  and  strong 
radicles,  like  those  of  -ZEsculus,  should  be  endowed 
with  less  sensitiveness  than  more  delicate  ones ;  for 
the  former  would  be  able  by  the  force  of  their  growth 
to  overcome  any  slight  obstacle. 

After  a  radicle,  which  has  been  deflected  by  some 
stone  or  root  from  its  natural  downward  course, 
reaches  the  edge  of  the  obstacle,  geotropism  will  direct 
it  to  grow  again  straight  downward ;  but  we  know  that 
geotropism  acts  with  very  little  force,  and  here  another 
excellent  adaptation,  as  Sachs  has  remarked,*  comes 
into  play.  For  the  upper  part  of  the  radicle,  a  little 
above  the  apex,  is,  as  we  have  seen,  likewise  sensitive ; 
and  this  sensitiveness  causes  the  radicle  to  bend  like  a 
tendril  towards  the  touching  object,  so  that  as  it  rubs 
over  the  edge  of  an  obstacle,  it  will  bend  downwards  ; 
and  the  curvature  thus  induced  is  abrupt,  in  which 
respect  it  differs  from  that  caused  by  the  irritation  of 
one  side  of  the  tip.  This  downward  bending  coincides 
•with  that  due  to  geotropism,  and  both  will  cause  the 
root  to  resume  its  original  course. 

As  radicles  perceive  an  excess  of  moisture  in  the  air 
on  one  side  and  bend  towards  this  side,  we  may  infer 
that  they  will  act  in  the  same  manner  with  respect  to 
moisture  in  the  earth.  The  sensitiveness  to  moisture 


*  « Arbciten  Bot.  Inst.  Wiirzburg,'  Heft  iii.  p.  456. 


CHAP.  III.  SUMMARY  OF    CHAPTER.  199 

resides  in  the  tip,  which  determines  the  bonding  of 
the  upper  part.  This  capacity  perhaps  partly  accounts 
for  the  extent  to  which  drain-pipes  often  become 
choked  with  roots. 

Considering  the  several  facts  given  in  this  chapter, 
we  see  that  the  course  followed  by  a  root  through 
the  soil  is  governed  by  extraordinarily  complex  and 
diversified  agencies, — by  geotropism  acting  in  a 
different  manner  on  the  primary,  secondary,  and  ter- 
tiary radicles, — by  sensitiveness  to  contact,  different  in 
kind  in  the  apex  and  in  the  part  immediately  above 
the  apex,  and  apparently  by  sensitiveness  to  the 
varying  dampness  of  different  parts  of  the  soil. 
These  several  stimuli  to  movement  are  all  more 
powerful  than  geotropism,  when  this  acts  obliquely 
on  a  radicle,  which  has  been  deflected  from  its  perpen- 
dicular '  downward  course.  The  roots,  moreover,  of 
most  plants  are  excited  by  light  to  bend  either  to  or 
from  it ;  but  as  roots  are  not  naturally  exposed  to  the 
light,  it  is  doubtful  whether  this  sensitiveness,  which  is 
perhaps  only  the  indirect  result  of  the  radicles  being 
highly  sensitive  to  other  stimuli,  is  of  any  service  to 
the  plant.  The  direction  which  the  apex  takes  at  each 
successive  period  of  the  growth  of  a  root,  ultimately 
determines  its  whole  course  ;  it  is  therefore  highly 
important  that  the  apex  should  pursue  from  the  first 
the  most  advantageous  direction ;  and  we  can  thus 
understand  why  sensitiveness  to  geotropism,  to  contact 
and  to  moisture,  all  reside  in  the  tip,  and  why  the  tip 
determines  the  upper  growing  part  to  bend  either 
from  or  to  the  exciting  cause.  A  radicle  may  be 
compared  with  a  burrowing  animal  such  as  a  mole, 
which  wishes  to  penetrate  perpendicularly  down  into 
the  ground.  By  continually  moving  his  head  from 
side  to  side,  or  circumnutating,  he  will  feel  any  stone 


200  SUMMARY  OF  CHAPTER.  CHAP.  III. 

or  other  obstacle,  as  well  as  any  difterence  in  the 
hardness  of  the  soil,  and  he  will  turn  from  that  side ; 
if  the  earth  is  damper  on  one  than  on  the  other  side 
he  will  turn  thitherward  as  a  better  hunting-ground. 
Nevertheless,  after  each  interruption,  guided  by  the 
sense  of  gravity,  he  will  be  able  to  recover  his  down- 
ward course  and  to  burrow  to  a  greater  depth. 


CHAP.  IV.  CIRCUMNUTATION.  201 


CHAPTEK  IV. 

THE  GlRCUMNUTATING  MOVEMENTS  OF  THE  SEVERAL  PARTS  OF 
MATURE  PLANTS. 

Circumnutation  of  stems :  concluding  remarks  on— Circumnutation  of 
stolons :  aid  thus  afforded  in  winding  amongst  the  stems  of  sur- 
rounding plants — Circumnutation  of  flower-stems — Circumnutation 
of  Dicotyledonous  leaves — Singular  oscillatory  movement  of  leaves 
of  Dionsea — Leaves  of  Cannabis  sink  at  night — Leaves  of  Gymno- 
sperms — Of  Monocotvledons — Cryptogams — Concluding  remarks 
on  the  circumnutation  of  leaves  :  generally  rise  in  the  evening  and 
sink  in  the  morning. 

WE  have  seen  in  the  first  chapter  that  the  stems  of  all 
seedlings,  whether  hypocotyls  or  epicotyls,  as  well  as 
the  cotyledons  and  the  radicles,  are  continually  cir- 
cumnutating — that  is,  they  grow  first  on  one  side  and 
then  on  another,  such  growth  being  probably  preceded 
by  increased  turgescence  of  the  cells.  As  it  was 
unlikely  that  plants  should  change  their  manner  of 
growth  with  advancing  age,  it  seemed  probable  that 
the  various  organs  of  all  plants  at  all  ages,  as  long  as 
they  continued  to  grow,  would  be  found  to  circum- 
nutate,  though  perhaps  to  an  extremely  small  extent. 
As  it  was  important  for  us  to  discover  whether  this 
was  the  case,  we  determined  to  observe  carefully  a 
certain  number  of  plants  which  were  growing  vigor- 
ously, and  which  were  not  known  to  move  in  any 
manner.  We  commenced  with  stems.  Observations 
of  this  kind  are  tedious,  and  it  appeared  to  us  that  it 
would  be  sufficient  to  observe  the  stems  in  about  a 
score  of  genera,  belonging  to  widely  distinct  families 
and  inhabitants  of  various  countries.  Several  plants 


202  CIRCUMNUTATION  OF  STEMS.  CHAP.  IV. 

were  selected  which,  from  being  woody,  or  for  other 
reasons,  seemed  the  least  likely  to  circumnutate.  The 
observations  and  the  diagrams  were  made  in  the 
manner  described  in  the  Introduction.  Plants  in  pots 
were  subjected  to  a  proper  temperature,  and  whilst 
being  observed,  were  kept  either  in  darkness  or  were 
feebly  illuminated  from  above.  They  are  arranged 
in.  the  order  adopted  by  Hooker  in  Le  Maout  and 
Decaisne's  '  System  of  Botany.'  The  number  of  the 
family  to  which  each  genus  belongs  is  appended,  as 
this  serves  to  show  the  place  of  each  in  the  series. 

(1.)  Iberia  umbdlata  (Cruciferae,  Fam.  14). — The  movement  of 
the  stem  of  a  young  plant,  4  inches  in  height,  consisting  of 
four  internodes  (the  hypocotyl  included)  besides  a  large  bud 

Fig  70. 


Ibcris  umbellata :  circumnutation  of  stem  of  young  plant,  traced  from 
8.30  A.M.  Sept.  13th  to  same  hour  on  following  morning.  Distance  of 
summit  of  stem  beneath  the  horizontal  glass  7'6  inches.  Diagram 
reduced  to  half  of  original  size.  Movement  as  here  shown  magnifie  1 
between  4  and  5  times. 

on  the  summit,  was  traced,  as  here  shown,  during  24  h. 
(Fig.  70).  As  far  as  we  could  judge  the  uppermost  inch  alone 
of  the  stem  circumnutated,  and  this  in  a  simple  manner.  The 
movement  was  slow,  and  the  rate  very  unequal  at  different 
times.  In  part  of  its  course  an  irregular  ellipse,  or  rather 
triangle,  was  completed  in  6  h.  30  m. 

(2.)  Brassica  oleracea  (CrucifersB). — A  very  young  plant,  bearing 
three  leaves,  of  which  the  longest  was  only  three-quarters  of  an 
inch  in  length,  was  placed  under  a  microscope,  furnished  with 
an  eye-piece  micrometer  and  the  tip  of  the  largest  leaf  was 


CHAP.  IV.  CIRCUMNUTATION   OF  STEMS.  203 

found  to  be  in  constant  movement.  It  crossed  five  divisions  of 
the  micrometer,  that  is,  -^th  of  an  inch,  in  6  m.  20  s.  There 
could  hardly  be  a  doubt  that  it  was  the  stem  which  chiefly 
moved,  for  the  tip  did  not  get  quickly  out  of  focus ;  and  this 
would  have  occurred  had  the  movement  been  confined  to  the 
leaf,  which  moves  up  or  down  in  nearly  the  same  vertical  plane. 

(3.)  Linum  usitatissimum  (Linese,  Fam.  39). — The  stems  of  this 
plant,  shortly  before  the  flowering  period,  are  stated  by  Fritz 
Mliller  ('  Jenaische  Zeitschrift,'  B.  v.  p.  137)  to  revolve,  or 
circumnutate. 

(4.)  'Pelargonium  zonale  (Geraniacese,  Fam.  47). — A  young 
plant,  7s  inches  in  height,  was  observed  in  the  usual  manner ; 
but,  in  order  to  see  the  bead  at  the  end  of  the  glass  filament 

Fig.  71. 


Pelargonium  zonale:  circumnutation  of  stem  of  young  plant,  feebly  illu- 
minated from  above.  Movement  of  bead  magnified  about  11  times  ; 
traced  on  a  horizontal  glass  from  noon  on  March  9th  to  8  A.M.  on 
the  llth. 

and  at  the  same  time  the  mark  beneath,  it  was  necessary  to  cut 
off  three  leaves  on  one  side.  We  do  not  know  whether  it  was 
owing  to  this  cause,  or  to  the  plant  having  previously  become 
bent  to  one  side  through  heliotropism,  but  from  .the  morning  of 
the  7th  of  March  to  10.30  P.M.  on  the  8th,  the  stem  moved 
a  considerable  distance  in  a  zigzag  line  in  the  same  general 
direction.  During  the  night  of  the  8th  it  moved  to  some 
distance  at  right  angles  to  its  former  course,  and  next  morning 
(9th)  stood  for  a  time  almost  still.  At  noon  on  the  9th  a  new 
tracing  was  begun  (see  Fig.  71),  which  was  continued  till  8  A.M. 
on  the  llth.  Between  noon  on  the  9th  and  5  P.M.  on  the  10th 
(i.e.  in  the  course  of  29  h.),  the  stem  described  a  circle.  This 
plant  therefore  circumnutates,  but  at  a  very  slow  rate,  and  to  a 
small  extent. 

(5.)  Tropceolum  mafus  (?)  (dwarfed  var.  called  Tom  Thumb) ; 
(Geraniaceso,  Fam.  47). — The  species  of  this  genus  climb  by  the 


204: 


CIKCUMXUTATION   OF   STEMS. 


CHAP.  IV. 


aid  of  their  sensitive  petioles,  but  some  of  them  also  twine 
round  supports ;  but  even  these  latter  species  do  not  begin  to 
circumnutate  in  a  conspicuous  manner  whilst  young.  The 

Fig.  72. 


TropcBolum  majus(?):  circumnutation  of  stem  of  young  plant,  traced  on  a 
horizontal  glass  from  9  A.M.  Dec.  26th  to  10  A.M.  on  27th.  Movement 
of  bead  magnified  about  5  times,  and  here  reduced  to  half  of  original 
scale. 

variety  here  treated  of  has  a  rather  thick  stem,  and  is  so  dwarf 
that  apparently  it  does  not  climb  in  any  manner.  We  there- 
fore wished  to  ascertain  whether  the  stem  of  a  young  plant, 

consisting  of  two  in- 
ternodes,  together  3 -2 
inches  in  height,  cir- 
cumnutated.  It  was 
observed  during  25  h., 
and  we  see  in  Fig.  72 
that  the  stem  moved  in 
a  zigzag  course,  indicat- 
ing circumnutation. 

(6.)  Trifolium  resupi- 
natum  (Leguminosae, 
Fam.  75).  —  \Vhen  we 
treat  of  the  sleep  of 
plants,  we  shall  see  that 
the  stems  in  several 
Leguminous  genera,  for 
instance,  those  of  Hedy- 
sarum,  Mimosa,  Meli- 
lotus,  &c.,  which  are  not 
climbers,  circumnutate 
in  aconspicuousmanner. 
We  will  here  give  only  a  single  instance  (Fig.  73),  showing 
the  circumnutation  of  the  stem  of  a  large  plant  of  a  clover, 
Trifolium  resupinatum.  In  the  course  of  7  h.  the  stem  changed 


Trifolium  res'tpinatum  :  circumnutation  of 
stem,  traced  on  vertical  glass  from  9.30 
A.M.  to  4.30  P.M.  Nov.  3rd.  Tracing  not 
greatly  magnified,  reduced  to  half  of 
original  size.  Plant  feebly  illuminated 
from  above. 


CHAP.  IV. 


CIRCUMNUTATION    OF   STEMS. 


205 


its  course  greatly  eight  times  and  completed  three  irregular 
circles  or  ellipses.  It  therefore  circumnutated  rapidly.  Somr 
of  the  lines  run  at  right  angles  to  one  another. 


Fig.  74. 


Rubus  (hyboid)  :  circumuutation  of  stem,  traced  on  horizontal  glass,  from 
4  P.M.  March  14th  to  8.30  A.M.  16th.  Tracing  much  magnified,  re- 
duced to  half  of  original  size.  Plant  illuminated  feebly  from  above. 

(7.)  Rubus  idceus  (hybrid)  (Kosacese,  Fam.  76). — As  we  hap- 
pened to  have  a  young  plant,  11  inches  F-  75 
in  height  and  growing  vigorously, 
which  had  been  raised  from  a  cross 
between  the  raspberry  (Rubus  idceus') 
and  a  North  American  Eubus,  it  was 
observed  in  the  usual  manner.  During 
the  morning  of  March  14th  the  stem 
almost  completed  a  circle,  and  then 
moved  far  to  the  right.  At  4  P.M.  it 
reversed  its  course,  and  now  a  fresh 
tracing  was  begun,  which  was  con- 
tinued during  40£  h.,  and  is  given  in 
Fig.  74.  We  here  have  well-marked 
circumnutation. 

(8.)  Deutzia  gracilis  (Saxifrageze, 
Fam.  77). — A  shoot  on  a  bush  about 
18  inches  in  height  was  observed.  The 
bead  changed  its  course  greatly  eleven  j)eutzi 
times  in  the  course  of  10  h.  30  m. 
(Fig.  75),  and  there  could  be  no 
doubt  about  the  circumnutation  of  the 
stem. 

(9.)  Fuchsia  (greenhouse  van,  with 
large  flowers,  probably  a  hybrid)  (Ona- 
grariese,  Fam.  100). — A  young  plant, 
15  inches  in  height,  was  observed  during  nearly  48  h. 


zia  grooms :  circumnu- 
tation of  stem,  kept  in 
darkness,  traced  on  hori- 
zontal glass,  from  8.30 
A.M.  to  7  P.M.March  20th. 
Movement  of  bead  origin- 
ally magnified  about  20 
times,  here  reduced  to 
half  scale. 


The 


OIEOUMNUTATION  OF  8TEMH.          <'.m-.  IV. 


ll.rri,|,i|.llll>lli;'     fif'lin-     <   I    I   '      /C)     |'IV«-.<;     ||,i-     |,rr,        ;,|        |  ,;,  I'l  |C||  III  fM, 
Jil:-l       I  llOWH       Mi.i.1       l.ll(l        I   I-  HI       .-II'  -Ulnl,  nl.,1.  .1,       I)  .....  :•!, 

alowly. 

Klg,  70. 


Fwhrit  (ffiwlftn  vnr.)  s  elrcumnuUtlon  of  »Um,  koptin  dnrkiw**,  trm-.-i  m 

horl«mtt]  fli   rfrons  »  :i"  A.M.  i.,  v  i-  M   MH-I.  !oih     M<.  fim-nt  of 
bond  ori^lniilty  iuii^tiilii»l  i\l.«,i.i    \<>  imn-N,  horo  rwJm  •  -I  '" 


(10.)    Ctiretta  tptri<i''iimiiHH*  (irnrilim   var,,  somoUmoH   c!ill««l 


whioh  wan  growing  vigoroUHly  from  liaving  Ixwn  n  •ni->\i-«l  a 
f«w  dayH  boforo  from  tlio  gn.'nnliaiiMo  to  the  hot-houno,  was 
olworvo(i  \viih  oipeclal  intoroHl,  IIM  it  nc(im«l  •  iiiii--  PK-IMM. 

lli:il  lh«  ,  •  1«  in  \\..iili|  i-iri-iiiiiniil;i.l«-.  'l'li«-  lii'iilicliCH  nn  ll;il,(,r 
H:I.|H  •Ililnrin  ,  Iml.  Hum-  of  II..  in  lire  1  1  1  nr  nl.  n  111  icdion,  wilh 

l.llO     tliniU    ni<l«M     liollo  \\ril     (.lit  A      I'TilllcIl     ol'     II,  ,        l;ill.i-|         llll.|M-, 

'.»  mi-ill  .  111  i<  in-ill  mid  Ji  in  <liainot(5r,  waH  Hi«.  en  i'«r  «''• 

li"ii,UM  1()HH  IJ|M  I.V  I"  I-IM  -1111111111.  il«-  HI.-III  :i  II  .I-  Mil,  ,1111  l.i.ilirli. 
Tim  liiMV.-iii.nl  of  tin-  l.ciul  ill,  tin-  m.  I  ••!'  Hi.-  :-lii.  :.::  lil;i  tin  -nl  , 

nitixr-.i  ID  tin-  i.iiiiiinii.  <>r  iii«-  branch,  WM  traced  (A,Fig.  77) 

IIMIII    :»-'.''.  A.M.  tO  4.80  I'.M.  MI,    N,,v.    ^.'Inl,   (lunnr.    \\lu<-li    tnm-    it, 
il       mm  ii-    1-1.  ;iM\       i       IIMM  <  »ii     tin-.    '.Mlli    iiiii.lln-r 

WliM  limdn  (H(!0  I1,  i.  :m.  I  tin-  l.riLil  on  tin.  cUj  •  -li  m  -.('(I  iti 
miir::«i  oll<-ii,r,  ni;il.ni!'  in  H  li.  \\lnit,  liiii>  !>«•  con:  nil  -n-i|  II.M  fmir 
i-lli|..:i  ::,  U  illi  tin  n  I  .Iill<  l'i  nlly  ilin-rli  il.  'I  In-  |  ••  •.  ltl"ii 

nl'    l.lir    i.li-in      ;iinl       il  .    r.iiiiini  -n.  -m:-      .-Miu-f-.e     M|I      tin-    I'ullnu  in-- 

ML.  I  Mill)'  .    :iM-     lll.i    \\  !!'(!     HllOWtl.  'M      !•••     MM    ll'.lllil.     lli:i.l      Hi'.* 

Ix.ilii   II,     Illiill^.ll     ll.|i|n  III  Hi"     (jllltc     i  .l.ll<  «l  ,     hill      HIM 


'.    IV. 


•   IK<   I   MM  TATION    OK    KTKMS. 


MM:  amount,  of  niovin<  ill,  during  UK;  timo  WUH   v«;r.y  hinsi.ll, 
rather  !<:;•;',  Until  UH:  ..'J.li  of  an  inrli. 

Pi*.  77. 


//  i'^V* »/» 

A 


tpeciooiiiimut  t  clrcumnutation  of  rtetn,  illuminuM  from  ahov«, 
tnwid  on  a  horizontal  (?!'**«,  '"  A  (l"'"  '•'  A-M-  l"  4-;{°  •'•»*.  on  Nov. 

93rd  |    ;./,<!    in     11    from    X.,'IO  A.M.   on    l.hi-    '/111.    I-.    H,  A.M.  on    In- 

MOV.  i,  mi.  .,/'  it,.-  i,.-.-i.i  in  i;  I,,:.  '„,),.  -I  abovi  N  tin 


(11.)  y/w/w«  /'/'/  (Anilm<™  ,  I  arn  111;.  'J  In:  :.l«:i/i  ii-;  known 
io  \,<;  ;i.|»ln:li«i!.j-(»|.ii-.,  iin<l  W)V(;ml  Willing*  prrowni;'  in  si.  |i'.t,  in 
Ui<:  f'K  <  nlj'/u  •<;  |,«  .c;u/i«:  hcnl  in  Ui<:  nil'ldlo  of  tl)<!  HIlfllfrM.f  "I. 
rij'.lil.  :inj'J«:;i  fio/n  Ul<5  li^ht.  Oil  Hfspt.  2tld  Konic  of  UK  c  I'm. 

i«l   up  K<>  UH  t<>  Mliirul  vorti(;ttlly,  and  wfin-,  pla/:<-,(i  lx;forc 
not  Ih  <';u-il,    window;    hut    to    our    mirpn:;<;    Uj<;y    w«;r<j    now 


'   <   towanJK    tli«:    li-'Jit,   JUJ'J    l.linir    r.onr    '     l<'in;'     !/.i.'-''l    on    ;i. 
lion/.o/i!;i.|    K\tLHH,  WttH  HtrOD^Iy   '/A^'/.'.ty,.       hunnj'    III';    0    Ml-  - 

•l;iyH  UK  y   r.irciii/iiiutaU'd   over  UK;   Hurn';  i.jiiall   npjir.u  at  a 
i.low  ratx:,  hut  UK  r«;  roiiM  l>o  no  •!•,  i!    UM  ir  rn''u/ii/nit;i. 

fciOO<      'Ih':  platltH  WOW  kept  CXar.Uy  in  tin:  ;.:ini«-  pla'-r.  |n:for«:  UK: 
window,    and    ;i.ll<  ;     an   interval    of    15    (layH     the    Ht<:inn   W';ro 
•gain  Observed  during  '2  d  ;•..>•;»  and  Uicir  lUOVeraentH  tra<x;<J,  and 
10 


208 


CIRCUMNUTATION   OF   STEMS. 


CHAP.  IV. 


they  were  found  to  be  still  circumnutating,  but  on  a  yet  smaller 
scale. 

(12.)  Gazania  ringens  (Composite,  Fam.  122).  —  The  circum- 
nutation  of  the  stem  of  a  young  plant,  7  inches  in  height,  as 
measured  to  the  tip  of  the  highest  leaf,  was  traced  during 
33  h.,  and  is  shown  in  the  accompanying  figure  (Fig.  78).  Two 

Fig.  78. 


Gazania  rinyens :  circunmutation  of  stem  traced  from  9  A.M.  March  21st 
to  6  P.M.  on  22nd;  plant  kept  in  darkness.  Movement  of  bead  at  the 
close  of  the  observations  magnified  34  times,  here  reduced  to  half  the 
original  scale. 

main  lines  may  be  observed  running  at  nearly  right  angles  to 
two  other  main  lines;  but  these  are  interrupted  by  small 
loops. 

(13.)  Azalea  Indica  (Ericineae,  Fam.  128).— A  bush  21  inches 
in  height  was  selected  for  observation,  and  the  circumnutation 
of  its  leading  shoot  was  traced  during  26  h.  40  m.,  as  shown 
in  the  following  figure  (Fig.  79). 

(14.)  Plumbago  Capensis  (Plumbaginese,  Fam.  134).— A  small 
lateral  branch  which  projected  from  a  tall  freely  growing  bush, 
at  an  angle  of  35°  above  the  horizon,  was  selected  for  obser- 
vation. For  the  first  11  h.  it  moved  to  a  considerable  distance 
in  a  nearly  straight  line  to  one  side,  owing  probably  to  its 
having  been  previously  deflected  by  the  light  whilst  standing  in 
the  greenhouse.  At  7.20  P.M.  on  March  7th  a  fresh  tracing  was 
begun  and  continued  for  the  next  43  h.  40  m.  (see  Fig.  8U). 
During  the  first  2  h.  it  followed  nearly  the  same  direction  as 
before,  and  then  changed  it  a  little;  during  the  night  it 
moved  at  nearly  right  angles  to  its  previous  course.  Next 


CHAP.  IV 


CIRCUMNUTATION   OF   STEMS 


209 


day  (8th)  it  zigzagged  greatly,  and  on  the  9th  moved  irregu- 
larly round  and  round  a  small  circular  space.  By  3  P.M.  on 
the  9th  the  figure  had  become  so  complicated  that  no  more  dots 
could  be  made ;  but  the  shoot  continued  during  the  evening  of 
the  9th,  the  whole  of  the  10th,  and  the  morning  of  the  llth  to 


Fig.  79. 


Azalea  Indica :  circum nutation 
of  stem,  illuminated  from 
above,  traced  on  horizontal 
glass,  from  9.30  A.M.  March 
9th  to  12.10  P.M.  on  the  10th. 
But  on  the  morning  of  the 
10th  only  four  dots  were 
made  between  8.30  A.M. 
and  12.10  P.M.,  both  hours 
included,  so  that  the  circum- 
nutation  is  not  fairly  repre- 
sented in  this  part  of  the 
diagram.  Movement  of  the 
bead  here  magnified  about 
30  times. 


Fig.  80. 


Plumbago  Capensis :  circumnu- 
tation  of  tip  of  a  lateral 
branch,  traced  on  horizontal 
glass,  from  7.20  P.M.  on 
March  7th  to  3  P.M.  on  the 
9th.  Movement  of  bead 
magnified  13  times.  Plant 
feebly  illuminated  from 
above. 


circumnutate  over  the  same  small  space,  which  was  only  about 
the  i^th  of  an  inch  ('97  mm.)  in  diameter.  Although  this 
branch  circumnutated  to  a  very  small  extent,  yet  it  changed  its 
course  frequently.  The  movements  ought  to  have  been  more 
magnified. 

(15.)  Aloysia  citriodora  (Terbenacese,  Fam.  173). — The  follow- 
ing figure  (Fig.  81)  gives  the  movements  of  a   shoot  during 

P 


210 


CIRCUMNUTATION   OF   STEMS. 


CHAP.  IV. 


31  h.  40  m.,  and  shows  that  it  circumnutated.    The  bush 
15  inches  in  height. 

Fig.  8.1. 


Aloysia  citrlodora :  circumnutation  of  stem,  traced  from  8.20  A.M.  on  March 
22nd  to  4  P.M.  on  23rd.  Plant  kept  in  darkness.  Movement  magnified 
about  40  times. 

(16.)  Verbena  melindres  (?)  (a  scarlet-flowered  herbaceous  var.) 
(Verbenaceae). — A  shoot  8  inches  in  height  had  been  laid  hori- 
zontally, for  the  sake  of  observing  its  apogeotropism,  and  the 
terminal  portion  had  grown  vertically  upwards  for  a  length  of 
li  inches.  A  glass  filament,  with  a  bead  at  the  end,  was  fixed 

Fig.  82  . 


Verbena  melindres:  circumnutation  of  stem  in  darkness,  traced  on  vertical 
glass,  from  5.30  P.M.  on  June  5th  to  11  A.M.  June  7th.  Movement  of 
bead  magnified  9  times. 

upright  to  the  tip,  and  its  movements  were  traced  during 
41  h.  30  m.  on  a  vertical  glass  (Fig.  82).  Under  these  circum- 
stances the  lateral  movements  were  chiefly  shown ;  but  as  the 
lines  from  side  to  side  are  not  on  the  same  level,  the  shoot 


GHAP.  IV.  CIRCUMNUTATION  OF   STEMS.  211 

must  have  moved  in  a  plane  at  right  angles  to  that  of  the  lateral 
movement,  that  is,  it  must  have  circumnutated.  On  the  next  day 
(6th)  the  shoot  moved  in  the  course  of  16  h.  four  times  to  the  right, 
and  four  times  to  the  left ;  and  this  apparently  represents  the 
formation  of  four  ellipses,  so  that  each  was  completed  in  4  h. 

(17.)  Ceratopliyllum  demersum  (Ceratophyllese,  Fam.  220). — An 
interesting  account  of  the  movements  of  the  stem  of  this  water- 
plant  has  been  published  by  M.  E.  Eodier.*  The  movements  are 
confined  to  the  young  internodes,  becoming  less  and  less  lower 
down  the  stem ;  and  they  are  extraordinary  from  their  amplitude. 
The  stems  sometimes  moved  through  an  angle  of  above  200°  in 
6  h.,  and  in  one  instance  through  220°  in  3  h.  They  generally 
bent  from  right  to  left  in  the  morning,  and  in  an  opposite  direc- 
tion in  the  afternoon  ;  but  the  movement  was  sometimes  tempo- 
rarily reversed  or  quite  arrested.  It  was  not  affected  by  light. 
It  does  not  appear  that  M.  Kodier  made  any  diagram  on  a  hori- 
zontal plane  representing  the  actual  course  pursued  by  the 
apex,  but  he  speaks  of  the  "  branches  executing  round  their 
axes  of  growth  a  movement  of  torsion."  From  the  particulars 
above  given,  and  remembering  in  the  case  of  twining  plants  and 
of  tendrils,  how  difficult  it  is  not  to  mistake  their  bending  to  all 
points  of  the  compass  for  true  torsion,  we  are  led  to  believe  that 
the  stems  of  this  Ceratophyllum  circumnutate,  probably  in  the 
shape  of  narrow  ellipses,  each  completed  in  about  26  h.  The 
following  statement,  however,  seems  to  indicate  something 
different  from  ordinary  circumnutation,  but  we  cannot  fully 
understand  it.  M.  Kodier  says :  "  II  est  alors  facile  de  voir  que 
le  mouvement  de  flexion  se  produit  d'abord  dans  les  merithalles 
superieurs,  qu'il  se  propage  ensuite,  en  s'amoindrissant  du  haut 
en  bas-,  tandis  qu'au  contraire  le  mouvement  de  redressement 
commence  par  la  partie  inferieure  pour  se  terminer  a  la  partie 
superieure  qui,  quelquefois,  peu  de  temps  avant  de  se  relever 
tout  a  fait,  forme  avec  1'axe  un  angle  tres  aigu." 

(18.)  Coniferce. — Dr.  Maxwell  Masters  states  ('  Journal  Linn. 
Soc./  Dec.  2nd,  1879)  that  the  leading  shoots  of  many  Coniferse 
during  the  season  of  their  active  growth  exhibit  very  remark- 
able movements  of  revolving  nutation,  that  is,  they  circumnu- 
tate, "We  may  feel  sure  that  the  lateral  shoots  whilst  growing 
would  exhibit  the  same  movement  if  carefully  observed. 


*  'Comptes  Rendus,'   April  30th.    1877.      Also  a  second   notice 
published  separately  in  Bourdeaux*  Nov.  12th,  1877. 


212  CIRCUMNUTATION   OF   STEMS.  CHAP.  IV. 

(19.)  Lilium  auratum  (Fam.  Liliacese).— The  circumnutation 

Fig.  83. 


Lilium  auratum:  circumnutation  of  a  stem  in  darkness,  traced  on  a  horizontal 
glass,  from  8  A.M.  on  March  14th  to  8.35  A.M.  on  16th.  But  it  should 
be  noted  that  our  observations  were  interrupted  between  6  P.M.  on  the 
14th  and  12.15  P.M.  on  15th,  and  the  movements  during  this  interval 
of  18  h.  15m.  ai-e  represented  by  a  long  broken  line.  Diagram  reduced 
to  half  original  scale. 

of  the  stem  of  a  plant  21  inches  in  height  is  represented  in  the 
above  figure  (Fig.  h3). 

Fig.  48. 


Cyperus  alternifolius :  circumnutation  of  stem,  illuminated  from  above, 
traced  on  horizontal  glass,  from  9.45  A.M.  March  9th  to  9P.M.  on  10th. 
The  stem  grew  so  rapidly  whilst  being  observed,  that  it  was  not  possible 
to  estimate  how  much  its  movements  were  magnified  in  the  tracing. 

(20.)    Cyperus    aUernifolius    (Fam.    Cyperacese.)  —  A    glass 


CHAP.  IV.  CIKCUMNUTATION   OF   STEMS.  213 

filament,  with  a  bead  at  the  end,  was  fixed  across  the  summit 
of  a  young  stem  10  inches  in  height,  close  beneath  the  crown  of 
elongated  leaves.  On  March  8th,  between  12.20  and  7.20  P.M., 
the  stem  described  an  ellipse,  open  at  one  end.  On  the  follow- 
ing day  a  new  tracing  was  begun  (Fig.  84),  which  plainly  shows 
that  the  stem  completed  three  irregular  figures  in  the  course  of 
35  h  15  m. 

Concluding  Remarks  on  the  Circumnuiation  of  Stems. — 
Any  one  who  will  inspect  the  diagrams  now  given,  and 
will  bear  in  mind  the  widely  separated  position  of  the 
plants  described  in  the  series,  — remembering  that  we 
have  good  grounds  for  the  belief  that  the  hypocotyls 
and  epicotyls  of  all  seedlings  circumnutate, — not 
forgetting  the  number  of  plants  distributed  in  the 
most  distinct  families  which  climb  by  a  similar  move- 
ment,— will  probably  admit  that  the  growing  stems 
of  all  plants,  if  carefully  observed,  would  be  found 
to  circumnutate  to  a  greater  or  less  extent.  When 
we  treat  of  the  sleep  and  other  movements  of  plants, 
many  other  cases  of  circumnutating  stems  will  be 
incidentally  given.  In  looking  at  the  diagrams,  we 
should  remember  that  the  stems  were  always  growing, 
so  that  in  each  case  the  circumnutating  apex  as  it 
rose  will  have  described  a  spire  of  some  kind.  The 
dots  were  made  on  the  glasses  generally  at  intervals 
of  an  hour,  or  hour  and  a  half,  and  were  then  joined 
by  straight  lines.  If  they  had  been  made  at  intervals 
of  2  or  3  minutes,  the  lines  would  have  been  more 
curvilinear,  as  in  the  case  of  the  tracks  left  on  the 
smoked  glass-plates  by  the  tips  of  the  circumnutating 
radicles  of  seedling  plants.  The  diagrams  generally 
approach  in  form  to  a  succession  of  more  or  less 
irregular  ellipses  or  ovals,  with  their  longer  axes 
directed  to  different  points  of  the  compass  during  the 
same  day  or  on  succeeding  days.  The  stems  ttyere- 


214  CIRCUMNUTATION  OF   STOLONS.        CHAP.  IV. 

fore,  sooner  or  later,  bend  to  all  sides;  but  after  a 
stern  has  bent  in  any  one  direction,  it  commonly 
bends  back  at  first  in  nearly,  though  not  quite,  the 
opposite  direction  ;  and  this  gives  the  tendency  to 
the  formation  of  ellipses,  which  are  generally  narrow, 
but  not  so  narrow  as  those  described  by  stolons  and 
leaves.  On  the  other  hand,  the  figures  sometimes 
approach  in  shape  to  circles.  Whatever  the  figure 
may  be,  the  course  pursued  is  often  interrupted  by 
zigzags,  small  triangles,  loops,  or  ellipses.  A  stern 
may  describe  a  single  large  ellipse  one  day,  and 
two  on  the  next.  With  different  plants  the  com- 
plexity, rate,  and  amount  of  movement  differs 
much.  The  stems,  for  instance,  of  Iberis  and  Azalea 
described  only  a  single  large  ellipse  in  24  h. ; 
whereas  those  of  the  Deutzia  made  four  or  five  deep 
zigzags  or  narrow  ellipses  in  11 J  h.,  and  those  of  the 
Trifolium  three  triangular  or  quadrilateral  figures 
in7h. 

ClRCUMNUTATIOlSr   OF   STOLONS  OR  KlJNNERS. 

Stolons  consist  of  much  elongated,  flexible  branches, 
which  run  along  the  surface  of  the  ground  and  form 
roots  at  a  distance  from  the  parent-plant.  They  are 
therefore  of  the  same  homological  nature  as  stems ; 
and  the  three  following  cases  may  be  added  to  the 
twenty  previously  given  cases. 

Frayaria  (cultivated  garden  var.) :  Rosacece. — A  plant  growing 
in  a  pot  had  emitted  a  long  stolon ;  this  was  supported  by  a 
stick,  so  that  it  projected  for  the  length  of  several  inches  hori- 
zontally. A  glass  filament  bearing  two  minute  triangles  of 
paper  was  affixed  to  the  terminal  bud,  which  was  a  little  up- 
turned ;  and  its  movements  were  traced  during  21  h.,  as  shown 
in  Fig.  85.  In  the  course  of  the  first  12  h.  it  moved  twice  up 
and  twice  down  in  somewhat  zigzag  lines,  and  no  doubt  tra- 
velled in  the  same  manner  during  the  night.  On  the  following 


CHAP.  IV.         CIRCUMNUTATION  OF   STOLONS. 


215 


morning  after  an  interval  of  20  h.  the  apex  stood  a  little  higher 
lhan  it  did  at  first,  and  this  shows  that  the  stolon  had  not  been 

Fig.  85. 


10°pm.S 


J0°4,5'q.m 
18™ 


:  circumnutation  of  stolon,   kept  in  darkness,  traced  on  vertical 
glass,  from  10.45  A.M.  May  18th  to  7.45  A.M.  on  19th. 


acted  on  within  this  time  by  geotropism;*   nor  had  its  own 
weight  caused  it  to  bend  downwards. 

On  the  following  morning  (19th)  the  glass  filament  was 
detached  and  refixed  close  behind  the  bud,  as  it  appeared  pos- 
sible that  the  circumnutation  of  the  terminal  bud  and  of  the 
adjoining  part  of  the  stolon  might  be  different.  The  movement 
was  now  traced  during  two  consecutive  days  (Fig.  86).  During 
the  first  day  the  filament  travelled  in  the  course  of  14  h.  30  m. 
five  times  up  and  four  times  down,  besides  some  lateral  move- 
ment. On  the  20th  the  course  was  even  more  complicated,  and 
can  hardly  be  followed  in  the  figure  ;  but  the  filament  moved  in 
16  h.  at  least  five  times  up  and  five  times  down,  with  very  little 


*  Dr.  A.  B.  Frank  states  ('  Die 
Naturliche  wagerechte  Richtung 
von  Pflanzentheilen,'  1S70,  p.  20) 
that  the  stolons  of  this  plant  are 


acted  on  by  geotropism,  but  only 
after  a  considerable  interval  of 
time. 


216 


CIRCUMNUTATION   OF   STOLONS.         CHAP.  IV. 


Fig.  86. 


lateral  deflection.  The  first  and  last  dots  made  on  this  second 
day,  viz.,  at  7  A.M.  and  11  P.M.,  were  close  together,  showing* 
that  the  stolon  had  not  fallen  or  risen.  Nevertheless,  by  com- 
paring its  position  on 
the  morning  of  the  19th 
and  21st,  it  is  obvious 
that  the  stolon  had  sunk ; 
and  this  may  be  attri- 
buted to  slow  bending 
down  either  from  its  own 
weight  or  from  geotro- 
pism. 

During  a  part  of  the  20th 
an  orthogonal  tracing  was 
made  by  applying  a  cube 
of  wood  to  the  vertical 
glass  and  bringing  the 
apex  of  the  stolon  at  suc- 
cessive periods  into  a  line 
with  one  edge;  a  dot 
being  made  each  time  on 
the  glass.  This  tracing 
therefore  represented  very 
nearly  the  actual  amount 
of  movement  of  the  apex ; 
and  in  the  course  of  9  h. 
the  distance  of  the  ex- 
treme dots  from  one  an- 
other was  '45  inch.  By 
the  same  method  it  was 
ascertained  that  the  apex 
moved  between  7  A.M.  on 
8>°a.m.21?  the  20th  and  8  A.M.  on  the 

Fragaria  :  circumnutation  of  the  same  stolon   21st  a  distance  of  "82  inch, 
as  in  the  last  figure,  observed  in  the  same        A  younger  and  shorter 

^  stolon  was  supported  so 
that  it  projected  at  about 
45°  above  the  horizon,  and  its  movement  was  traced  by  the 
same  orthogonal  method.  On  the  first  day  the  apex  soon 
rose  above  the  field  of  vision.  By  the  next  morning  it  had 
sunk,  and  the  course  pursued  was  now  traced  during  14  h. 
30  m.  (Fig.  87).  The  amount  of  movement  was  almost  the  same, 


CHAP.  IV.        CIRCUMNUTATION   OF   STOLONS.  217 

from  side  to  side  as  up  and  down ;  and  differed  in  this  respect 
remarkably  from  the  movement  in  the  previous  cases.  During 
the  latter  part  of  the  day,  viz.,  between  3  and  10.30  P.M.,  the 

Fig.  87. 


Warn. 

Fragwia:  circumnutation  of  another   and   younger   stolon,  traced  from 
8  A.M.  to  10.30  P.M.     Figure  reduced  to  one-half  of  original  scale. 

actual  distance  travelled  by  the  apex  amounted  to  1*15  inch; 
and  in  the  course  of  the  whole  day  to  at  least  2*67  inch.  This 
is  an  amount  of  movement  almost  comparable  with  that  of 
some  climbing  plants.  The  same  stolon  was  observed  on  the 
following  day,  and  now  it  moved  in  a  somewhat  less  complex 
manner,  in  a  plane  not  far  from  vertical.  The  extreme  amount 
of  actual  movement  was  T55  inch  in  one  direction,  and  '6  inch 
in  another  direction  at  right  angles.  During  neither  of  these 
days  did  the  stolon  bend  downwards  through  geotropism  or  its 
own  weight. 

Four  stolons  still  attached  to  the  plant  were  laid  on  damp 
sand  in  the  back  of  a  room,  with  their  tips  facing  the  north-east 
windows.  They  were  thus  placed  because  De  Vries  says  *  that 
they  are  apheliotropic  when  exposed  to  the  light  of  the  sun ;  but 
we  could  not  perceive  any  effect  from  the  above  feeble  degree  of 
illumination.  We  may  add  that  on  another  occasion,  late  in  the 
summer,  some  stolons,  placed  upright  before  a  south-west  window 


*  « Arbeiten  Bot.  Inst.,  Wurzburg,'  1872,  p.  434. 


218 


CIECUMNUTATION   OF   STOLONS.        CHAP   IV. 


on  a  cloudy  day,  became  distinctly  curved  towards  the  light,  and 
were  therefore  heliotropic.  Close  in  front  of  the  tips  of  the 
prostrate  stolons,  a  crowd  of  very  thin  sticks  and  the  dried 
haulms  of  grasses  were  driven  into  the  sand,  to  represent  the 
crowded  steins  of  surrounding  plants  in  a  state  of  nature.  This 
was  done  for  the  sake  of  observing  how  the  growing  stolons 
would  pass  through  them.  They  did  so  easily  in  the  course  of 
6  days,  and  their  circumnutation  apparently  facilitated  their 
passage.  When  the  tips  encountered  sticks  so  close  together 
that  they  could  not  pass  between  them,  they  rose  up  and  passed 
over  them.  The  sticks  and  haulms  were  removed  after  the 
passage  of  the  four  stolons,  two  of  which  were  found  to  have 
assumed  a  permanently  sinuous  shape,  and  two  were  still 
straight.  But  to  this  subject  we  shall  recur  under  Saxifraga. 

Saxifraya  sarmentosa  (Saxifragese). — A  plant  in  a  suspended 
pot  had  emitted  long  branched  stolons,  which  depended  like 


Saxifraya  sarmentosa:  circumnutation  of  an   inclined  stolon,  traced    iu 
darkness  on  a  horizontal  glass,  from  7.45  A.M.  April  18th  to  9  A.M.  on 
9th.     Movement  of  end  of  stolon  magnified  2-2  times. 

threads  on  all  sides.  Two  were  tied  up  so  as  to  stand  vertically, 
and  their  upper  ends  became  gradually  bent  downwards,  but  so 
slowly  in  the  course  of  several  days,  that  the  bending  was  pro- 
bably due  to  their  weight  and  not  to  geotropism.  A  glass  fila- 
ment with  little  triangles  of  paper  was  fixed  to  the  end  of  one  of 
these  stolons,  which  was  17£  inches  in  length,  and  had  already 
become  much  bent  down,  but  still  projected  at  a  considerable 
angle  above  the  horizon.  It  moved  only  slightly  three  times 
from  side  to  side  and  then  upwards;  on  the  following  day 


CHAP.  IV.         CIRCUMNUTATION   OF   STOLONS.  219 

the  movement  was  even  less.  As  this  stolon  was  so  long  we 
thought  that  its  growth  was  nearly  completed,  so  we  tried 
another  which  was  thicker  and  shorter,  viz.,  10i  inches  in  length. 
It  moved  greatly,  chiefly  upwards,  and  changed  its  course  five 
times  in  the  course  of  the  day.  During  the  night  it  curved  so 
much  upwards  in  opposition  to  gravity,  that  the  movement 
could  no  longer  be  traced  on  the  vertical  glass,  and  a  horizontal 
one  had  to  be  used.  The  movement  was  followed  during  the 
next  25  h.,  as  shown  in  Fig.  88.  Three  irregular  ellipses,  with 
their  longer  axes  somewhat  differently  directed,  were  almost 
completed  in  the  first  15  h.  The  extreme  actual  amount  of 
movement  of  the  tip  during  the  25  h.  was  *75  inch. 

Several  stolons  were  laid  on  a  flat  surface  of  damp  sand,  in  the 
same  manner  as  with  those  of  the  strawberry.  The  friction  of 
the  sand  did  not  interfere  with  their  circumnutation ;  nor  could 
we  detect  any  evidence  of  their  being  sensitive  to  contact.  In 
order  to  see  how  in  a  state  of  nature  they  would  act,  when 
encountering  a  stone  or  other  obstacle  on  the  ground,  short 
pieces  of  smoked  glass,  an  inch  in  height,  were  stuck  upright 
into  the  sand  in  front  of  two  thin  lateral  branches.  Their  tips 
scratched  the  smoked  surface  in  various  directions ;  one  made 
three  upward  and  two  downward  lines,  besides  a  nearly  hori- 
zontal one;  the  other  curled  quite  away  from  the  glass;  but 
ultimately  both  surmounted  the  glass  and  pursued  their  original 
course.  The  apex  of  a  third  thick  stolon  swept  up  the  glass  in  a 
curved  line,  recoiled  and  again  came  into  contact  with  it ;  it  then 
moved  to  the  right,  and  after  ascending,  descended  vertically ; 
ultimately  it  passed  round  one  end  of  the  glass  instead  of  over  it. 

Many  long  pins  were  next  driven  rather  close  together  into 
the  sand,  so  as  to  form  a  crowd  in  front  of  the  same  two  thin 
lateral  branches;  but  these  easily  wound  their  way  through 
the  crowd.  A  thick  stolon  was  much  delayed  in  its  passage ; 
at  one  place  it  was  forced  to  turn  at  right  angles  to  its  former 
course;  at  another  place  it  could  not  pass  through  the  pins, 
and  the  hinder  part  became  bowed ;  it  then  curved  upwards 
and  passed  through  an  opening  between  the  upper  part  of  some 
pins  which  happened  to  diverge ;  it  then  descended  and  finally 
emerged  through  the  crowd.  This  stolon  was  rendered  perma- 
nently sinuous  to  a  slight  degree,  and  was  thicker  where  sinuous 
than  elsewhere,  apparently  from  its  longitudinal  growth  having 
been  checked. 

Cotyledon  umbilicus  (Crassulaceje). — A  plant  growing  in  a  pan 


220 


CIRCUMNUTATION   OF   STOLONS.         CHAP.  IV. 


of  damp  moss  had  emitted  2  stolons,  22  and  20  inches  in  length. 
One  of  these  was  supported,  so  that  a  length  of  4£  inches  pro- 
jected in  a  straight  and  horizontal  line,  and  the  movement 
of  the  apex  was  traced.  The  first  dot  was  made  at  9.10  A.M.  ; 

Fig.  89. 


Cotyledon  umbilicus:  circumnutation  of  stolon,  traced  from  11.15  A.M. 
Aug.  25th  to  11  A.M.  27th.  Plant  illuminated  from  above.  The 
terminal  internode  was  '25  inch  in  length,  the  penultimate  2 '25,  aud 
the  third  3'0  inches  in  length.  Apex  of  stolon  stood  at  a  distance  of 
5  *  75  inches  from  the  vertical  glass  ;  but  it  was  not  possible  to  ascertain 
how  much  the  tracing  was  magnified,  as  it  was  not  known  how  great 
a  length  of  the  internode  circumnutated. 

the  terminal  portion  soon  began  to  bend  downwards  and  con- 
tinued to  do  so  until  noon.  Therefore  a  straight  line,  very 
nearly  as  long  as  the  whole  figure  here  given  (Fig.  89),  was  first 
traced  on  the  glass ;  but  the  upper  part  of  this  line  has  not  been 
copied  in  the  diagram.  The  curvature  occurred  in  the  middle 


CHAP.  IV.          CIKCUMNUTATION  OF  STOLONS.  221 

of  the  penultimate  internode;  and  its  chief  seat  was  at  the 
distance  of  li  inch  from  the  apex;  it  appeared  due  to  the 
weight  of  the  terminal  portion,  acting  on  the  more  flexible 
part  of  the  internode,  and  not  to  geotropism.  The  apex  after 
thus  sinking  down  from  9.10  A.M.  to  noon,  moved  a  little  to  the 
left;  it  then  rose  up  and  circumnutated  in  a  nearly  vertical 
plane  until  10.35  P.M.  On  the  following  day  (26th)  it  was  ob- 

Fig.  90. 


ITa.m  *^» 

Cotyledon  umbilicus :  circumnutation  and  downward  movement  of  another 
stolon,  traced  on  vertical  glass,  from  9.11  A.M.  Aug.  25th  to  11  A.M.  27th. 
Apex  close  to  glass,  so  that  figure  but  little  magnified,  and  here  reduced 
to  two-thirds  of  original  size. 

served  from  6.40  A.M.  to  5.20  P.M.,  and  within  this  time  it  moved 
twice  up  and  twice  down.  On  the  morning  of  the  27th  the  apex 
stood  as  high  as  it  did  at  11.30  A.M.  on  the  25th.  Nor  did  it 
sink  down  during  the  28th,  but  continued  to  circurnnutate  about 
the  same  place. 
Another  stolon,  which  resembled  the  last  in  almost  every 


222  CIRCUMNUTATION  OF  STOLONS.          CHAP.  IV. 

respect,  was  observed  daring  the  same  two  days,  but  only  two 
inches  of  the  terminal  portion  was  allowed  to  project  freely  and 
horizontally.  On  the  25th  it  continued  from  9.10  A.M.  to  1.30  P.M. 
to  bend  straight  downwards,  apparently  owing  to  its  weight 
(Fig.  90);  but  after  this  hour  until  10.35  P.M.  it  zigzagged. 
This  fact  deserves  notice,  for  we  here  probably  see  the  combined 
effects  of  the  bending  down  from  weight  and  of  circumnutation. 
The  stolon,  however,  did  not  circumnutate  when  it  first  began 
to  bend  down,  as  may  be  observed  in  the  present  diagram,  and 
as  was  still  more  evident  in  the  last  case,  when  a  longer  portion 
of  the  stolon  was  left  unsupported.  On  the  following  day 
(26th)  the  stolon  moved  twice  up  and  twice  down,  but  still  con- 
tinued to  fall ;  in  the  evening  and  during 'the  night  it  travelled 
from  some  unknown  cause  in  an  oblique  direction. 

We  see  from  these  three  cases  that  stolons  or 
runners  circumnutate  in  a  very  complex  manner.  The 
lines  generally  extend  in  a  vertical  plane,  and  this 
may  probably  be  attributed  to  the  effect  of  the  weight 
of  the  unsupported  end  of  the  stolon ;  but  there  is 
always  some,  and  occasionally  a  considerable,  amount 
of  lateral  movement.  The  circumnutation  is  so  great 
in  amplitude  that  it  may  almost  be  compared  with 
that  of  climbing  plants.  That  the  stolons  are  thus 
aided  in  passing  over  obstacles  and  in  winding  between 
the  stems  of  the  surrounding  plants,  the  observations 
above  given  render  almost  certain.  If  they  had  not 
circumnutated,  their  tips  would  have  been  liable  to 
have  been  doubled  up,  as  often  as  they  met  with 
obstacles  in  their  path  ;  but  as  it  is,  they  easily  avoid 
them.  This  must  be  a  considerable  advantage  to  the 
plant  in  spreading  from  its  parent-stock;  but  we  are 
far  from  supposing  that  the  power  has  been  gained 
by  the  stolons  for  this  purpose,  for  circumnutation 
seems  to  be  of  universal  occurrence  with  all  growing 
parts ;  but  it  is  not  improbable  that  the  amplitude 
of  the  movement  may  have  been  specially  increased 
for  this  purpose. 


CHAP.  IV.      CIRCUMNUTATION  OF  FLOWEK-STEMS.     223 


ClRCUMNUTATION    OF    FLOWER-STEMS. 

We  did  not  think  it  necessary  to  make  any  special 
observations  on  the  circumnutation  of  flower-stems, 
these  being  axial  in  their  nature,  like  stems  or  stolons  ; 
but  some  were  incidentally  made  whilst  attending 
to  other  subjects,  and  these  we  will  here  briefly  give. 
A  few  observations  have  also  been  made  by  other 
botanists.  These  taken  together  suffice  to  render  it 
probable  that  all  peduncles  and  sub-peduncles  cir- 
cumnutate  whilst  growing. 

Oxalis  carnosa.— The  peduncle  which  springs  from  the  thick 
and  woody  stem  of  this  plant  bears  three  or  four  sub-peduncles . 

Fig.  91. 


Oxalis  carnosa :  flower-stem,  feebly  illuminated  from  above,  its  circximnuta 
tion  traced  from  9  A.M.  April  13th  to  9  A.M.  15th.     Summit  of  flower 
8  inches  beneath  the  horizontal   glass.      Movement  probably  magnified 
about  6  times. 

A  filament  with  little  triangles  of  paper  was  fixed  within  the 
calyx  of  a  flower  which  stood  upright.  Its  movements  were 
observed  for  48  h. ;  during  the  first  half  of  this  time  the  flower 
was  fully  expanded,  and  during  the  second  half  withered.  The 
figure  here  given  (Fig.  91)  represents  8  or  9  ellipses.  •  Although 
the  main  peduncle  circumnutated,  and  described  one  large  and 


224     CIKCUMNUTAT1ON  OF  FLOWER-STEMS       CHAP.  IV. 

two  smaller  ellipses  in  the  course  of  21  h.,  yet  the  chief  seat  of 
movement  lies  in  the  sub-peduncles,  which  ultimately  bend 
vertically  downwards,  as  will  be  described  in  a  future  chapter. 
The  peduncles  of  Oxalis  acetosdla  likewise  bend  downwards,  and 
afterwards,  when  the  pods  are  nearly  mature,  upwards ;  and  this 
is  effected  by  a  circumnutating  movement. 

It  may  be  seen  in  the  above  figure  that  the  flower-stem  of 
O.carnosa  circumnutated  during  two  days  about  the  same  spot. 
On  the  other  hand,  the  flower-stem  of  0.  sensitiva  undergoes  a 
strongly  marked,  daily,  periodical  change  of  position,  when  kept 
at  a  proper  temperature.  In  the  middle  of  the  day  it  stands 
vertically  up,  or  at  a  high  angle ;  in  the  afternoon  it  sinks,  and 
in  the  evening  projects  horizontally,  or  almost  horizontally, 
rising  again  during  the  night.  This  movement  continues  from 
the  period  when  the  flowers  are  in  bud  to  when,  as  we  believe, 
the  pods  are  mature :  and  it  ought  perhaps  to  have  been  included 
amongst  the  so-called  sleep-movements  of  plants.  A  tracing 
was  not  made,  but  the  angles  were  measured  at  successive  periods 
during  one  whole  day;  and  these  showed  that  the  movement 
was  not  continuous,  but  that  the  peduncle  oscillated  up  and 
down.  We  may  therefore  conclude  that  it  circumnutated.  At 
the  base  of  the  peduncle  there  is  a  mass  of  small  cells,  forming 
a  well- developed  pulvinus,  which  is  exteriorly  coloured  purple 
and  hairy.  In  no  other  genus,  as  far  as  we  know,  is  the  peduncle 
furnished  with  a  pulvinus.  The  peduncle  of  0.  Ortegesii  behaved 
differently  from  that  of  0.  sensitiva,  for  it  stood  at  a  less  angle 
above  the  horizon  in  the  middle  of  the  day,  than  in  the  morning 
or  evening.  By  10.20  P.M.  it  had  risen  greatly.  During  the 
middle  of  the  day  it  oscillated  much  up  and  down. 

Trifolium  subterraneum. — A  filament  was  fixed  vertically  to 
the  uppermost  part  of  the  peduncle  of  a  young  and  upright 
flower-head  (the  stem  of  the  plant  having  been  secured  to  a 
stick);  and  its  movements  were  traced  during  36  h.  Within 
this  time  it  described  (see  Fig.  92)  a  figure  which  represents  four 
ellipses;  but  during  the  latter  part  of  the  time  the  peduncle 
began  to  bend  downwards,  and  after  10.30  P.M.  on  the  24th  it 
curved  so  rapidly  down,  that  by  6.15  A.M.  on  the  25th  it  stood 
only  19°  above  the  horizon.  It  went  on  circumnutating  in  nearly 
the  same  position  for  two  da  vs.  Even  after  the  flower-heads 
have  buried  themselves  in  the  ground  they  continue,  as  will 
hereafter  be  shown,  to  circumnutate.  It  will  also  be  seen  in  the 
next  chapter  that  the  sub-peduncles  of  the  separate  flowers  of 


CHAP.  IV.     CIKCUMNUTATION  OF  FLOWER-STEMS.     225 

TrifoUum  repens  circumnutate  in  a  complicated  course  during 
several  days.    I  may  add  that  the  gynophore  of  Arachis  hypogwa, 


''40'a.m  23** 


TrifoUum  subterrancum :  main  flower-peduncle,  illuminated  from  above, 
circumnutation  traced  on  horizontal  glass,  from  8.40  A.M.  July  23rd 
to  10.30  P.M.  24th. 

which  looks  exactly  like  a  peduncle,  circumnutates  whilst  growing 
vertically  downwards,  in  order  to  bury  the  young  pod  in  the 
ground. 

The  movements  of  the  flowers  of  Cyclamen  Persicum  were  not 
observed ;  but  the  peduncle,  whilst  the  pod  is  forming,  increases 
much  in  length,  and  bows  itself  down  by  a  circumnutating 
movement.  A  young  peduncle  of  Maurandia  semperflorens, 
li  inch'  in  length,  was  carefully  observed  during  a  whole  day, 
and  it  made  42  narrow,  vertical,  irregular  and  short  ellipses, 
each  at  an  average  rate  of  about  2  h.  '25  m.  An  adjoining 
peduncle  described  during  the  same  time  similar,  though  fewer, 
ellipses.*  According  to  Sachs  f  the  flower-stems,  whilst  growing, 


*  *  The  Movements  and  Habits 
of  Climbing  Plants,'   2nd   edit., 


1875,  p.  68. 
t  '  Text-Book  of  Botany,'  1875, 


226  CIRCUMNUTATION  OF  LEAVES.        CHAP.  IV. 

of  many  plants,  for  instance,  those  of  Brassica  napus,  revolve  or 
circumnutate ;  those  of  Attium  porrum  bend  from  side  to  side, 
and,  if  this  movement  had  been  traced  on  a  horizontal  glass, 
no  doubt  ellipses  would  have  been  formed.  Fritz  Miiller  has 
described  *  the  spontaneous  revolving  movements  of  the  flower- 
stems  of  an  Alisma,  which  he  compares  with  those  of  a  climbing 
plant. 

We  made  no  observations  on  the  movements  of  the  different 
parts  of  flowers.  Morren,  however,  has  observed  f  in  the 
stamens  of  Sparmannia  and  Cereus  a  "  fremissement  spontane," 
which,  it  may  be  suspected,  is  a  circumnutating  movement. 
The  circumnutation  of  the  gynostemium  of  Stylidium,  as  de- 
scribed by  Gad,J  is  highly  remarkable,  and  apparently  aids  in 
the  fertilisation  of  the  flowers.  The  gynostemium,  whilst  spon- 
taneously moving,  comes  into  contact  with  the  viscid  labellum, 
to  which  it  adheres,  until  freed  by  the  increasing  tension  of  the 
parts  or  by  being  touched. 

We  have  now  seen  that  the  flower-stems  of  plants 
belonging  to  such  widely  different  families  as  the 
Cruciferae,  Oxalidae,  Leguminosae,  Primulacese,  Scro- 
phularinese,  Alismaceae,  and  Liliaceae,  circumnutate ; 
and  that  there  are  indications  of  this  movement  in 
many  other  families.  With  these  facts  before  us, 
bearing  also  in  mind  that  the  tendrils  of  not  a  few 
plants  consist  of  modified  peduncles,  we  may  admit 
without  much  doubt  that  all  growing  flower-stems 
circumnutate. 

CIRCUMNUTATION  OF  LEAVES  ::  DICOTYLEDONS. 

Several  distinguished  botanists,  Hofmeister,  Sachs, 
Pfeffer,  De  Vries,  Batalin,  Millardet,  &c.,  have  ob- 


p.  766.     Linnreus  and  Trevirunus  plies  oircumnutation. 
(according   to   Pfeffer,    'Die   Pe-  *    •  Jenaische   Zeitsch.,'    B.   v. 

riodischen  Bewegungen,'  &c.,    p.  p.  133. 

162)  state  that  the  flower-stalks  f  '  N.  Mem.  de  1'Acad.  R.  de 

of  many  plants  occupy  different  Bruxelles,'  torn.  xiv.  1841,  p.  3. 
positions  by  night  and  day,  and          J  « Sitzungbericht  des  bot.  Ve- 

we  shall  ste  in  the  chapter  on  reins   der  P.  Brandenburg,'  xxi 

the  Sleep  of  Plants  that  th:s  im-  p.  84. 


CHAP.  IV.  DICOTYLEDONS.  227 

served,  and  some  of  them  with  the  greatest  care,  the 
periodical  movements  of  leaves ;  but  their  attention 
has  been  chiefly,  though  not  exclusively,  directed  to 
those  which  move  largely  and  are  commonly  said  to 
sleep  at  night.  From  considerations  hereafter  to  be 
given,  plants  of  this  nature  are  here  excluded,  and 
will  be  treated  of  separately.  A's  we  wished  to  ascer- 
tain whether  all  young  and  growing  leaves  circumnu- 
tated,  we  thought  that  it  would  be  sufficient  if  we 
observed  between  30  and  40  genera,  widely  distributed 
throughout  the  vegetable  series,  selecting  some  un- 
usual forms  and  others  on  woody  plants.  All  the 
plants  were  healthy  and  grew  in  pots.  They  were 
illuminated  from  above,  but  the  light  perhaps  was  not 
always  sufficiently  bright,  as  many  of  them  were  ob- 
served under  a  skylight  of  ground-glass.  Except  in  a 
few  specified  cases,  a  fine  glass  filament  with  two  minute 
triangles  of  paper  was  fixed  to  the  leaves,  and  their 
movements  were  traced  on  a 
vertical  glass  (when  not  stated 
to  the  contrary)  in  the  manner 
already  described.  I  may  repeat 
that  the  broken  lines  represent 
the  nocturnal  course.  The  stem 
was  always  secured  to  a  stick,  ^ 
close  to  the  base  of  the  leaf 
under  observation.  The  ar- 
rangement Of  the  Species,  With  Sarraceniapurpw-ea:*™*- 

the  number  of  the  Family  ap-      nutation  of  young  pitcher, 

T     n      .       .,  '        ,1  traced  from  8  A.M.  July  3rd 

pended,  is  the  same  as  m  the      to  10.15  A.M.  4th.    Temp, 
case  of  stems.  17°-1 8°  c-  APex  of  Pitcher 

20   inches   from    glass,    so 
movement     greatly     mag- 
(1.)    Sarracenia    purpurea     (Sarra-     nified. 

cenesB,  Fam.   11).— A  young  leaf,  or 

pitcher,  85  inches  in  height,  with  the  bladder  swollen,  but  with 

the  hood  not   as  yet  open,  had  a  filament  fixed  transversely 


^    Fig.  93. 


228 


CIKCUMNUTATION   OF  LEAVES. 


CHAP.  IV. 


Fig.  94 


across  its  apex  ;  it  was  observed  for  48  h.,  and  during  the  whole 
of  this  time  it  circumnutated  in  a  nearly  similar  manner,  but 
to  a  very  small  extent.  The  tracing  given  (Fig.  93)  relates 
only  to  the  movements  during  the  first  26  h. 

(2.)  Glaucium  luteum  (Papave- 
raceae,  Fam.  12).— A  young  plant, 
bearing  only  8  leaves,  had  a  fila- 
ment attached  to  the  youngest  leaf 
but  one,  which  was  3  inches  in 
length,  including  the  petiole.  The 
circumnutating  movement  was 
traced  during  47  h.  On  both  days 
the  leaf  descended  from  before  7  A.M. 
until  about  11  A.M.,  and  then 
ascended  slightly  during  the  rest 
of  the  day  and  the  early  part  of 
the  night.  During  the  latter  part 
of  the  night  it  fell  greatly.  It  did 
not  ascend  so  much  during  the 
second  as  during  the  first  day,  and 
it  descended  considerably  lower  on 
the  second  night  than  on  the  first. 
This  difference  was  probably  due 
to  the  illumination  from  above 
having  been  insufficient  during  the 
two  days  of  observation.  Its  course 
during  the  two  days  is  shown  in 
Fig.  94. 

(3.)  Cramle  maritima  (Cruciferse, 
Fam.  14). — A  leaf  9  £  inches  in  length 
on  a  plant  not  growing  vigorously 
was  first  observed.  Its  apex  was 
in  constant  movement,  but  this 
could  hardly  be  traced,  from  being 
so  small  in  extent.  The  apex,  how- 
ever, certainly  changed  its  course  at 
least  6  times  in  the  course  of  14  h. 
A  more  vigorous  young  plant,  bear- 
ing only  4  leaves,  was  then  selected, 
and  a  filament  was  affixed  to  the 
midrib  of  the  third  leaf  from  the  base,  which,  with  the  petiole,  was 
6  inches  in  length.  The  leaf  stood  up  almost  vertically,  but  the  tip 


Glaucium  luteum :  circumnuta- 
tion  of  young  leaf,  traced 
from  9.30  A.M.  June  14th 
to  8.30  A.M.  16th.  Tracing 
not  much  magnified,  as  apex 
of  leaf  stood  only  5J  inches 
from  the  glass. 


CHAP.  IY. 


DICOTYLEDONS. 


229 


Fig.  95. 


was  deflected,  so  that  the  filament  projected  almost  horizontally, 

and  its  movements  were  traced  during  48  h.  on  a  vertical  glass, 

as  shown  in  the  accompanying  figure  (Fig.  95).   We  here  plainly 

see  that  the  leaf  was  con- 
tinually circumnutating ; 

but  the  proper  periodicity 

of  its  movements  was  dis- 
turbed by  its  being  only 

dimly   illuminated    from 

above  through  a  double 

skylight.     We  infer  that 

this  was  the  case,  because 

two  leaves  on  plants  grow- 
ing out  of  doors,  had  their 

angles  above  the  horizon 

measured  in  the  middle 

of  the  day  and  at  9  to 

about  10  P.M.  on  succes- 
sive nights,  and  they 

were  found  at  this  latter 

hour  to  have  risen  by  an 

average  angle  of  9°  above 

their    mid-day   position : 

on  the  following  morning 

they  fell  to  their  former 

position.    Now  it  may  be 

observed  in  the  diagram 

that  the  leaf  rose  during 

the  second  night,  so  that 

it  stood  at  6.40  A.M.  higher 

than  at  10.20  P.M.  on  the  Crambe  maritima :  circumnutation  of  leaf, 

disturbed  by  being  insufficiently  illumi- 
nated from  above,  traced  from  7.50  A.M. 
June  23rd  to  8  A.M.  25th.  Apex  of  leaf 
15J  inches  from  the  vertical  glass,  so  that 
the  tracing  was  much  magnified,  but  is 
here  reduced  to  one-fourth  of  original  scale. 

(4.)  Brassica  oleracea  (Cruciferse).—  Hofmeister  and  Batalin  * 
state  that  the  leaves  of  the  cabbage  rise  at  night,  and  fall  by 
day.  We  covered  a  young  plant,  bearing  8  leaves,  under  a  large 
bell-glass,  placing  it  in  the  same  position  with  respect  to  the 


preceding  night ;  and  this 
may  be  attributed  to  the 
leaf  adjusting  itself  to  the 
dim  light,  coming  exclu- 
sively from  above. 


Flora,'  1873,  p.  437 


230  CIRCUMNUTATION  OF  LEAVES/        CHAP.  IV. 

light  in  which  it  had  long  remained;  and  a  filament  was  fixed 
at  the  distance  of  *4  of  an  inch  from  the  apex  of  a  young  leaf 
nearly  4  inches  in  length.  Its  movements  were  then  traced 
during  three  days,  but  the  tracing  is  not  worth  giving.  The 
leaf  fell  during  the  whole  morning,  and  rose  in  the  evening  and 
during  the  early  part  of  the  night.  The  ascending  and  descend- 
ing lines  did  not  coincide,  so  that  an  irregular  ellipse  was  formed 
each  24  h.  The  basal  part  of  the  midrib  did  not  move,  as  was 
ascertained  by  measuring  at  successive  periods  the  angle  which 
it  formed  with  the  horizon,  so  that  the  movement  was  confined 
to  the  terminal  portion  of  the  leaf,  which  moved  through  an 
angle  of  11°  in  the  course  of  24  h.,  and  the  distance  travelled  by 
the  apex,  up  and  down,  was  between  •  8  and  •  9  of  an  inch. 

In  order  to  ascertain  the  effect  of  darkness,  a  filament  was 
fixed  to  a  leaf  5£  inches  in  length,  borne  by  a  plant  which  after 
forming  a  head  had  produced  a  stem.  The  leaf  was  inclined 
44°  above  the  horizon,  and  its  movements  were  traced  on  a 
vertical  glass  every  hour  by  the  aid  of  a  taper.  During  the 
first  day  the  leaf  rose  from  8  A.M.  to  10.40  P.M.  in  a  slightly 
zigzag  course,  the  actual  distance  travelled  by  the  apex  being 
•  67  of  an  inch.  During  the  night  the  leaf  fell,  whereas  it  ought 
to  have  risen ;  and  by  7  A.M.  on  the  following  morning  it  had 
fallen  -23  of  an  inch,  and  it  continued  falling  until  9.40  A.M.  It 
then  rose  until  10.50  P.M.,  but  the  rise  was  interrupted  by  one 
considerable  oscillation,  that  is,  by  a  fall  and  re-ascent.  During 
the  second  night  it  again  fell,  but  only  to  a  very  .short  distance, 
and  on  the  following  morning  re-ascended  to  a  very  short 
distance.  Thus  the  normal  course  of  the  leaf  was  greatly 
disturbed,  or  rather  completely  inverted,  by  the  absence  of 
light ;  and  the  movements  were  likewise  greatly  diminished  in 
amplitude. 

We  may  add  that,  according  to  Mr.  A.  Stephen  Wilson,*  the 
young  leaves  of  the  Swedish  turnip,  which  is  a  hybrid  between 
B.  oleracea  and  rapa,  draw  together  in  the  evening  so  much 
"  that  the  horizontal  breadth  diminishes  about  30  per  cent,  of 
the  daylight  breadth."  Therefore  the  leaves  must  rise  con- 
siderably at  night. 

(5.)  Dianthus  caryophyllus   (Caryophyllese,  Fain.   26).  —  The 


*  '  Trans.  Bot.  Soc.  Edinburgh,'  see  Darwin,  '  Animals  and  Plants 
vol.  xiii.  p.  32.  With  respect  to  under  Domestication,'  2nd  edit, 
the  origin  of  the  Swedish  turnip.  vol.  i.  p.  344. 


CHAP.  IV.  DICOTYLEDONS.  231 

terminal  shoot  of  a  young  plant,  growing  very  vigorously,  was 
selected  for  observation.  The  young  leaves  at  first  stand  up 
vertically  and  close  together,  but  they  soon  bend  outwards  and 
downwards,  so  as  to  become  horizontal,  and  often  at  the  same 
time  a  little  to  one  side.  A  filament  was  fixed  to  the  tip  of  a 
young  leaf  whilst  still  highly  inclined,  and  the  first  dot  was 
made  on  the  vertical  glass  at  8.30  A.M.  June  13th,  but  it  curved 
downwards  so  quickly  that  by  6.40  A.M.  on  the  following 
morning  it  stood  only  a  little  above  the  horizon.  In  Fig.  96 


Dianthus  caryophyllus :  circumnutation  of  young  leaf,  traced  from  10.15 
P.M.  June  13th  to  10.35  P.M.  16th.  Apex  of  leaf  stood,  at  the  close  of 
our  observations,  8|  inches  from  the  vertical  glass,  so  tracing  not 
greatly  magnified.  The  leaf  was  5£  inches  long.  Temp.  15£°-17£°  C. 

the  long,  slightly  zigzag  line  representing  this  rapid  downward 
course,  which  was  somewhat  inclined  to  the  left,  is  not  given ; 
but  the  figure  shows  the  highly  tortuous  and  zigzag  course, 
together  with  some  loops,  pursued  during  the  next  2  3  days. 
As  the  leaf  continued  to  move  all  the  time  to  the  left,  it  is 
evident  that  the  zigzag  line  represents  many  circumnutations. 

(6.)  Camellia  Japonica  (Camelliacese,  Fam.  32). — A  youngish 
leaf,  which  together  with  its  petiole  was  21  inches  in  length  and 
which  arose  from  a  side  branch  on  a  tall  bush,  had  a  filament 
attached  to  its  apex.  This  leaf  sloped  downwards  at  an  angle 
of  40°  beneath  the  horizon.  As  it  was  thick  and  rigid,  and  its 
11 


232 


CIHCUMNUTATION   OF   LEAVES. 


CIIAP.  IV. 


petiole  very  short,  much  movement  could  not  be  expected 


Fig.  97. 


Nevertheless,  the  apex  changed  its  course 
completely  seven  times  in  the  course  of 
11s  h.,  but  moved  to  only  a  very  small 
distance.  On  the  next  day  the  movement 
of  the  apex  was  traced  during  26  h.  20  m. 
(as  shown  in  Fig.  97),  and  was  nearly  of 
the  same  nature,  but  rather  less  complex. 
The  movement  seems  to  be  periodical,  for 
on  both  days  the  leaf  circumnutated  in  the 
forenoon,  fell  in  the  afternoon  (on  the  first 
day  until  between  3  and  4  P.M.,  and  on  the 
second  day  until  6  P.M.),  and  then  rose, 
falling  again  during  the  night  or  early 
morning. 

In  the  chapter  on  the  Sleep  of  Plants 
we  shall  see  that  the  leaves  in  several  Malvaceous  genera  sink 

Fig.  98. 


Camellia  Jaj>onica:  cir- 
cumnutation  of  leaf, 
traced  from  6.40 
A.M.  June  14th  to 
6.50  A.M.  15th. 
Apex  of  leaf  12 
inches  from  the  ver- 
tical glass,  so  figure 
considerably  mag- 
nified. Temp.  lt»°- 
16J°  C. 


Pelargonium  zonale  :  circumnutation  and  downward  movement  of  young 
leaf,  traced  from  9.30  A.M.  June  14th  to  6.30  P.M.  16th.  Apex  of  leaf 
9J  inches  from  the  vertical  glass,  so  figure  moderately  magnified. 
Temp.  15°-16J°  C. 

at  night;  and  as  they  often  do  not  then  occupy  a  vertical 
position,  especially  if  they  have  not  been  well  illuminated  during 


CHAP.  IV. 


DICOTYLEDONS. 


233 


Fig.  99 


the  day,  it  is  doubtful  whether  some  of  these  cases  ought  not 
to  have  been  included  in  the  present  chapter. 

(7.)  Pelargonium  zonale  (Geraniaceao,  Fain.  47).  —  A  young 
leaf,  li  inch  in  breadth,  with  its  petiole  1  inch  long,  borne  on 
a  young  plant,  was  observed  in  the  usual  manner  during  61  h. ; 
and  its  course  is  shown  in  the  preceding  figure  (Fig.  98). 
During  the  first  day  and  night  the  leaf  moved  downwards,  but 
circumnutated  between  10  A.M.  and  4.30  P.M.  On  the  second 
day  it  sank  and  rose  again,  but  between  10  A.M.  and  6  P.M.  it 
circumnutated  on  an  extremely  small  scale.  On  the  third  day 
the  circumnutation  was  more  plainly  marked. 

(8.)  Cissus  discolor  (Ampelidese,  Fam.  67). — A  leaf,  not  nearly 
full-grown,  the  third  from  the  apex  of 
a  shoot  on  a  cut-down  plant,  was 
observed  during  31  h.  30  m.  (see  Fig. 
99).  The  day  was  cold  (15°-16°  0.), 
and  if  the  plant  had  been  observed  in 
the  hot-house,  the  circumnutation, 
though  plain  enough  as  it  was,  would 
probably  have  been  far  more  con- 
spicuous. 

(9.)  Vicia  fdba  (Leguminosae,  Fam. 
75). — A  young  leaf,  3*1  inches  in 
length,  ^measured  from  base  of  petiole  to 
end  of  leaflets,  had  a  filament  affixed 
to  the  midrib  of  one  of  the  two  ter- 
minal leaflets,  and  its  movements  were 
traced  during  51£  h.  The  filament  fell 
all  morning  (July  2nd)  till  3  P.M.,  and 
then  rose  greatly  till  10.35  P.M.  ;  but 
the  rise  this  day  was  so  great,  com- 
pared with  that  which  subsequently 
occurred,  that  it  was  probably  due  in 
part  to  the  plant  being  illuminated 
from  above.  The  latter  part  of  the  course  on  July  2nd  is  alone 
given  in  the  following  figure  (Fig.  100).  On  the  next  day 
(July  3rd)  the  leaf  again  fell  in  the  morning,  then  circumnu- 
tated in  a  conspicuous  manner,  and  rose  till  late  at  night ;  but 
the  movement  was  not  traced  after  7.15  P.M.,  as  by  that  time  the 
filament  pointed  towards  the  upper  edge  of  the  glass.  During 
the  latter  part  of  the  night  or  early  morning  it  again  fell  in  the 
same  manner  as  before. 


Cissus  discolor ;  circumnu- 
tation of  leaf,  traced 
from  10.35  A.M.  May 
28th  to  6  P.M.  29th. 
Apex  of  leaf  8f  inches 
from  the  vertical  glass. 


234 


CIRCUMNUTATION   OF   LEAVES. 


CHAP.  IV. 


As  the  evening  rise  and  the  early  morning  fall  were  unusually 
large,  the  angle  of  the  petiole  above  the  horizon  was  measured 
at  the  two  periods,  and  the  leaf  was  found  to  have  risen  19° 

Fig.  100. 


Vicia  faba:  circumnutation  of  leaf,  traced  from  7.15  P.M.  July  2nd  to 
10.15  A.M.  4th.  Apex  of  the  two  terminal  leaflets  7J  inches  from  the 
vertical  ^lass.  Figure  here  reduced  to  two-thirds  of  original  scale. 
Temp.  17°-18°  C. 

between  12.20  P.M.  and  10.45  P.M.,  and  to  have  fallen  23°  30* 

between  the  latter  hour  and  10.20  A.M.  on  the  following  morning. 

The  main  petiole  was  now  secured  to  a  stick  close  to  the  base 


CHAP.  IV. 


DICOTYLEDONS. 


235 


of  the  two  terminal  leaflets,  which  were  1  '4  inch  in  length ;  and 
the  movements  of  one  of  them  were  traced  during  48  h.  (see 
Fig.  101).  The  course  pursued  is  closely  analogous  to  that  of 
the  whole  leaf.  The  zigzag  line  between  8.30  A.M.  and  3.30  P.M. 
on  the  second  day  represents  5  very  small  ellipses,  with  their 

Fig.  101. 


Yicia  faha:  circumnutation  of  one  of  the  two  terminal  leaflets,  the  main 
petiole  having  been  secured,  traced  from  10.40  A.M.  July  4th  to  10.30  A.M. 
6th.  Apex  of  leaflet  6|  inches  from  the  vertical  glass.  Tracing  here 
reduced  to  one-half  of  original  scale.  Temp.  16°-18°  C. 

longer  axes  differently  directed.  From  the^e  observations  it 
follows  that  both  the  whole  leaf  and  the  terminal  leaflets  undergo 
a  well-marked  daily  periodical  movement,  rising  in  the  evening 
and  falling  during  the  latter  part  of  the  night  or  early  morning ; 
whilst  in  the  middle  of  the  day  they  generally  circumnutate 
round  the  same  small  space: 


236 


CIKCUMNUTATION   OF   LEAVES.          CHAP.  IV. 


Fig  102. 


Fig.  103. 


(10.)  Acacia  retinoides  (LeguminosaB).— - The  movement  of  a 
young  phyllode,  2f  inches  in  length,  and  inclined  at  a  consider- 
able angle  above  the  horizon,  was  traced 
during  45  h.  30  m. ;  but  in  the  figure  here 
given  (Fig.  102),  its  circumnutation  is  shown 
during  only  21  h.  30  m.  During  part  of 
this  time  (viz.,  14  h.  30  m.)  the  phyllode 
described  a  figure  re- 
presenting 5  or  6 
small  ellipses.  The 
actual  amount  of 
movement  in  a  ver- 
tical direction  was  '3 
inch.  The  phyllode 
rose  considerably  be- 
tween 1.30  P.M.  and 
4  P.M.,  but  there  was 
no  evidence  on  either 
day  of  a  regular  pe- 
riodic movement. 

(11.)  Lupin-us  si/e- 
ciosus  (Leguminosce). 
— Plants  were  .raised 

from    seed  purchased  under  this  name. 

This  is  one  of  the  species  in  this  large 

genus,  the  leaves  of  which  do  not  sleep 

at  night.    The  petioles  rise  direct  from 

the  ground,  and  are  from  5  to  7  inches 

in  length.    A  filament  was  fixed  to  the 

midrib  of  one  of  the  longer  leaflets,  and 

the  movement  of  the  whole  leaf  was  traced, 

as  shown  in  Fig.  103.    In  the  course  of 

6  h.  30  m.  the  filament  went  four  times  up 

and  three  times  down.     A  new  tracing 

was   then  begun  (not  here  given),  and 

during  12  .l  h.  the  leaf  moved  eight  times   LuPinus  - 

cumnutation  of  leaf 

up    and    seven    times   down;   so  that  it 

described   7z  ellipses  in  this   time,  and 

this  is  an  extraordinary  rate  of  movement. 

The  summit  of  the  petiole  was  then  secured 

to  a  stick,  and  the  separate  leaflets  were  found  to  be  continually 

circumnutating.  » 


Acacia  retinoides :  cir- 
cumnutation of  a 
young  phyllode, 
traced  from  10.45 
A.M.  July  18th  to 
8.15  A.M.  19th. 
Apex  of  phyllode  9 
inches  from  the 
vertical  glass;  temp. 
16^°-17°  C. 


traced  on  vertical 
glass,  from  10.15A.M. 
to  5.45  P.M.;  i.e., 
during  6  h.  30  in. 


CHAP.  IV. 


DICOTYLEDONS. 


237 


Fig.  104. 


(12.)  Echeveria  stolunifera  (Crassulacese,  Fam.  84). — The  older 
leaves  of  this  plant  are  so  thick  and  fleshy,  and  the  young  ones 
so  short  and  broad,  that  it  seemed 
very  improbable  that  any  circum- 
nutation  could  be  detected.  A  fila- 
ment was  fixed  to  a  young  upwardly 
inclined  leaf,  '75  inch  in  length  and 
•28  in  breadth,  which  stood  on  the 
outside  of  a  terminal  rosette  of  leaves, 
produced  by  a  plant  growing  very 
vigorously.  Its  movement  was  traced 
during  3  days,  as  here  shown  (Fig. 
104).  The  course  was  chiefly  in  an 
upward  direction,  and  this  may  be 
attributed  to  the  elongation  of  the 
leaf  through  growth ;  but  we  see  that 
the  lines  are  strongly  zigzag,  and  that 
occasionally  there  was  distinct  cir- 
cumnutation,  though  on  a  very  small 
scale. 

(13.)  Bryophyllum  (vel  Calanchce) 
calycinuiti  (Crassulaceae).  —  Duval- 
Jouve  ('Bull.  Soc.  Bot.  de  France,' 
Feb.  14th,  1868)  measured  the  dis- 
tance between  the  tips  of  the  upper 
pair  of  leaves  on  this  plant,  with  the  result  shown  in  the  following 
Table.  It  should  be  noted  that  the  measurements  on  Dec.  2nd 
were  made  on  a  different  pair  of  leaves : — 


Nov.  16 

„     19 

Dec.     2 


We  see  from  this  Table  that  the  leaves  stood  considerably 
further  apart  at  2  P.M.  than  at  either  8  A.M.  or  7  P.M.  ;  and  this 
shows  that  they  rise  a  little  in  the  evening  and  fall  or  open 
in  the  forenoon. 

(14.)  Drosera  rot  undif alia  (Droseraceae,  Fain.  85).— The  move- 
ments of  a  young  leaf,  having  a  long  petiole  but  with  its  tentacles 
(01  gland-bearing  hairs)  as  yet  unfolded,  were  traced  during 
47  h.  15  m.  The  figure  (Fig.  105)  shows  that  it  circumnutated 
largely,  chiefly  in  a  vertical  direction,  making  two  ellipses  each 


Echeveria  stolonifera :  circum- 
nutation  of  leaf,  traced 
from  8.20  A.M.  June  25th 
to  8.45  A.M.  28th.  Apex 
of  leaf  12£  inches  from  the 
glass,  so  that  the  movement 
was  much  magnified;  temp. 
23°-24|°  C. 


8A.M. 

2  P.M. 

7  P.M. 

.    15  mm. 

.   25  mm. 

.      .      .     (?) 

*&     .   48    „ 

...   60    „ 

.      .      .   48  mm. 

22 

.      .      .   43    „ 

.    28    „ 

238 


CIRCUMNUTATIOX   OF   LEAVES. 


CHAP.  IV. 


Fig.  105. 


day.     On  both  days  the  leaf  began  to  descend  after  12  or 
1   o'clock,  and  continued  to  do  so  all  night,  though   to   a 

very  unequal  distance  on  the 
two  occasions.  We  therefore 
thought  that  the  movement 
was  periodic;  but  on  observ- 
ing three  other  leaves  during 
several  successive  days  and 
nights,  we  found  this  to  be  an 
error;  and  the  case  is  given 
merely  as  a  caution.  On  the 
third  morning  the  above  leaf 
occupied  almost  exactly  the 
same  position  as  on  the  first 
morning ;  and  the  tentacles 
by  this  time  had  unfolded 
sufficiently  to  project  at  right 
angles  to  the  blade  or  disc. 

The    leaves    as  they   grow 
older    generally    sink    more 
and  more  downwards.     The 
movement  of  an   oldish   leaf, 
the    glands    of    which    were 
still     secreting     freely,     was 
traced  for  24  h.,  during  which 
time  it   continued  to  sink  a 
little  in  a  slightly  zigzag  line. 
On  the  following  morning,  at 
Drosera  ratundifolw :  circumnutation      7    A.M.,   a   drop   of  a  Solution 
of  young  leaf ,  with  filament  fixed     of  '  carbonate   of  ammonia  (2 
to  back  of  blade,  traced  from  9.15  ,  . 

A.M.  June  7th  to  8.30  A.M.  June  &'  to  l  OZ'  of  Water>  was 
9th.  Figure  here  reduced  to  one-  placed  on  the  disc,  and  this 
half  original  scale.  blackened  the  glands  and  in- 

duced inflection  of  many  of  the  tentacles.  The  weight  of  the 
drop  caused  the  leaf  at  first  to  sink  a  little ;  but  immediately 
afterwards  it  began  to  rise  in  a  somewhat  zigzag  course,  and 
continued  to  do  so  till  3  P.M.  It  then  circumnutated  about 
the  same  spot  on  a  very  small  scale  for  21  h. ;  and  during  the 
next  21  h.  it  sank  in  a  zigzag  line  to  nearly  the  same  level 
which  it  had  held  when  the  ammonia  was  first  administered. 
By  this  time  the  tentacles  had  re-expanded,  and  the  glan.ds  had 
recovered  their  proper  colour.  We  thus  learn  that  an  old  leaf 


CHAP.  IV.  DICOTYLEDONS.  239 

circumnutates  on  a  small  scale,  at  least  whilst  absorbing  car- 
bonate of  ammonia  ;  for  it  is  probable  that  this  absorption  may 
stimulate  growth  and  thus  re-excite  circumnutation.  Whether 
the  rising  of  the  glass  filament  which  was  attached  to  the  back 
of  the  leaf,  resulted  from  its  margin  becoming  slightly  inflected 
(as  generally  occurs),  or  from  the  rising  of  the  petiole,  was  not 
ascertained. 

In  order  to  learn  whether  the  tentacles  or  gland-bearing  hairs 
circumnutate,  the  back  of  a  young  leaf,  with  the  innermost 
tentacles  as  yet  incurved,  was  firmly  cemented  with  shellac 
to  a  flat  stick  driven  into  compact  damp  argillaceous  sand. 
The  plant  was  placed  under  a  microscope  with  the  stage  re- 
moved and  with  an  eye- piece  micrometer,  of  which  each 
division  equalled  -5-^  of  an  inch.  It  should  be  stated  that  as 
the  leaves  grow  older  the  tentacles  of  the  exterior  rows  bend 
outwards  and  downwards,  so  as  ultimately  to  become  deflected 
considerably  beneath  the  horizon.  A  tentacle  in  the  second 
row  from  the  margin  was  selected  for  observation,  and  was 
found  to  be  moving  outwards  at  a  rate  of  ¥£o  of  an  inch  in 
20  m.,  or  ^  of  inch  in  1  h.  40  m. ;  but  as  it  likewise  moved 
from  side  to  side  to  an  extent  of  above  T^  of  inch,  the  move- 
ment was  probably  one  of  modified  circumnutation.  A  tentacle 
on  an  old  leaf  was  next  observed  in  the  same  manner.  In 
15  m.  after  being  placed  under  the  microscope  it  had  moved 
about  YoVo  °f  an  inch.  During  the  next  7k  h.  it  was  looked  at 
repeatedly,  and  during  this  whole  time  it  moved  only  another 
To1™  °f  an  mcn  5  and  tnis  small  movement  may  have  been  due 
to  the  settling  of  the  damp  sand  (on  which  the  plant  rested), 
though  the  sand  had  been  firmly  pressed  down.  We  may  there- 
fore conclude  that  the  tentacles  when  old  do  not  circumnutate  ; 
yet  this  tentacle  was  so  sensitive,  that  in  23  seconds  after  its 
gland  had  been  merely  touched  with  a  bit  of  raw  meat,  it  began 
to  curl  inwards.  This  fact  is  of  some  importance,  as  it  appa- 
rently shows  that  the  inflection  of  the  tentacles  from  the  stimulus 
of  absorbed  animal  matter  (and  no  doubt  from  that  of  contact 
•  with  any  object)  is  not  due  to  modified  circumnutation. 

(15.)  Dioncea  miHscipula  (Droseraceae). — It  should  be  premised 
that  the  leaves  at  an  early  stage  of  their  development  have  the 
two  lobes  pressed  closely  together.  These  are  at  first  directed 
back  towards  the  centre  of  the  plant ;  but  they  gradually  rise  up 
and  soon  stand  at  right  angles  to  tbe  petiole,  and  ultimately  in 
nearly  a  straight  line  with  it.  A  young  leaf,  which  with  the 


2-iO 


CIKCUMNUTAT10N    OF   LEAVES. 


CHAP.  IV. 


Fig.  106. 


petiole  was  only  1'2  inch  in  length,  had  a  filament  fixed  exter- 
nally along  the  midrib  of  the  still  closed  lobes,  which  projected 
at  right  angles  to  the  petiole.  In  the  evening  this  leaf  com- 
pleted an  ellipse  in  the  course  of  2  h.  On 
the  following  day  (Sept.  25th)  its  move- 
ments were  traced  during  22  h. ;  and  we 
see  in  Fig.  106  that  it  moved  in  the  same 
general  direction,  due  to  the  straightening 
of  the  leaf,  but  in  an  extremely  zigzag  line. 
This  line  represents  several  drawn-out  or 
modified  ellipses.  There  can  therefore  be 
no  doubt  that  this  young  leaf  circunmu- 
tated. 

A  rather  old,  horizontally  extended 
leaf,  with  a  filament  attached  along  the 
under  side  of  the  midrib,  was  next 
observed  during  7  h.  It  hardly  moved, 
but  when  one  of  its  sensitive  hairs 
was  touched,  the  blades  closed,  though 
not  very  quickly.  A  new  dot  was  now 
made  on  the  glass,  but  in  the  course  of 
14  h.  20  m.  there  was  hardly  any  change 
in  the  position  of  the  filament.  We  may 
therefore  infer  that  an  old  and  only 
moderately  sensitive  leaf  does  not  circum- 
nutate  plainly ;  but  we  shall  soon  see 
that  it  by  no  means  follows  that  such 
a  leaf  is  absolutely  motionless.  We  may 
further  infer  that  the  stimulus  from  a 
touch  does  not  re-excite  plain  circumnu- 
tation. 

Another  full-grown  leaf  had  a  filament 
attached  externally  along  one  side  of  the 
midrib  and  parallel  to  it,  so  that  the  fila- 
ment would  move  if  the  lobes  closed.  It 
should  be  first  stated  that,  although  a  touch  on  one  of  the  sensi- 
tive hairs  of  a  vigorous  leaf  causes  it  to  close  quickly,  often 
almost  instantly,  yet  when  a  bit  of  damp  meat  or  some  solution 
of  carbonate  of  ammonia  is  placed  on  the  lobes,  they  close  so 
slowly  that  generally  24  h.  is  required  for  the  completion  of  the 
act.  The  above  leaf  was  first  observed  for  2  h.  30  m.,  and  did 
not  circnmnntate,  but  it  ought  to  have  been  observed  for  a 


Doticea  mttfdpti/b :  cir- 
oumnutation  of  a 
young  and  expanding 
leaf,  traced  on  a  hori- 
zontal glass  in  dark- 
ness, from  noon  Sept. 
24th  to  10  A.M.  25th. 
Apex  of  leaf  13J 
inches  from  the  glass, 
so  tracing  consider- 
ably magnified. 


CHAP.  IV.  DICOTYLEDONS  241 

longer  period  ;  although,  as  we  have  seen,  a  young  leaf  com- 
pleted a  fairly  large  ellipse  in  2  h.  A  drop  of  an  infusion  of 
raw  meat  was  then  placed  on  the  leaf,  and  within  2  h.  the  glass 
filament  rose  a  little  ;  and  this  implies  that  the  lobes  had  begun 
to  close,  and  perhaps  the  petiole  to  rise.  It  continued  to  rise 
with  extreme  slowness  for  the  next  8  h.  30  m.  The  position  of 
the  pot  was  then  (7.15  P.M.,  Sept.  24th)  slightly  changed  and 
an  additional  drop  of  the  infusion  given,  and  a  new  tracing 
was  begun  (Fig.  107).  By  10.50  P.M.  the  filament  had  risen 
only  a  little  more,  and  it  fell  during  the  night.  On  the  follow- 
ing morning  the  lobes  were  closing  more  quickly,  and  by  5  P.M. 
it  was  evident  to  the  eye  that  they  had  closed  considerably  ;  by 
8.48  P.M.  this  was  still  plainer,  and  by  10.45  P.M.  the  marginal 
spikes  were  interlocked.  The  leaf  fell  a  little  during  the  night, 
and  next  morning  (25th)  at  7  A.M.  the  lobes  were  completely 
shut.  The  course'  pursued,  as  may  be  seen  in  the  figure,  was 


Fig.  107. 


Dioncc-i  mu.^ci}>ula  :  closure  of  ihe  lobes  and  circumnutation  of  a  full-grown 
leaf,  whilst  absorbing  an  infusion  of  raw  meat,  traced  in  darkness,  from 
7.15  P.M.  Sept.  24th  to  9  A.M.  26th.  Apex  of  leaf  8£  inches  from  the 
vertical  glass.  Figure  here  reduced  to  two-thirds  of  original  scale. 

strongly  zigzag,  and  this  indicates  that  the  closing  of  the  lobes 
was  combined  with  the  circumnutation  of  the  whole  leaf; 
and  there  cannot  be  much  doubt,  considering  how  motionless 
the  leaf  was  during  2  h.  30  m.  before  it  received  the  infusion, 
that  the  absorption  of  the  animal  matter  had  excited  it  to 
circumnutate.  The  leaf  was  occasionally  observed  for  the  next 
four  days,  but  was  kept  in  rather  too  cool  a  place  ;  nevertheless, 
it  continued  to  circumnutate  to  a  small  extent,  and  the  lobes 
remained  closed. 

It  is  sometimes  stated  in  botanical  works  that  the  lobes  close 
or  sleep  at  night  ;  but  this  is  an  error.  To  test  the  statement, 
very  long  glass  filaments  were  fixed  inside  the  two  lobes  of 
three  leaves,  and  the  distances  between  their  tips  were  measured 
in  the  middle  of  the  day  and  at  night  ;  but  no  difference  could 
be  detected. 

The  previous  observations  relate  to  the  movements  of  the 
whole  leaf,  but  the  lobes  move  independently  of  the  petiole,  and 


242  C1KCUMNUTATION   OF  LEAVES.         CHAP.  IV. 

seem  to  be  continually  opening  and  shutting  to  a  very  small 
extent.  A  neirly  full -grown  leaf  (afterwards  proved  to  be 
highly  sensitive  to  contact)  stood  almost  horizontally,  so  that 
by  driving  a  long  thin  pin  through  the  foliaceous  petiole  close 
to  the  blade,  it  was  rendered  motionless.  The  plant,  with 
a  little  triangle  of  paper  attached  to  one  of  the  marginal  spikes, 
was  placed  under  a  microscope  with  an  eye-piece  micrometer, 
each  division  of  which  equalled  ^^  of  an  inch.  The  apex  of 
the  paper-triangle  was  now  seen  to  be  in  constant  slight  move- 
ment ;  for  in  4  h.  it  crossed  nine  divisions,  or  -5^  of  an  inch, 
and  after  ten  additional  hours  it  moved  back  and  had  crossed 
•sfo  in  an  opposite  direction.  The  plant  was  kept  in  rather 
too  cool  a  place,  and  on  the  following  day  it  moved  rather  less, 
namely,  -^  in  3  h.,  and  ¥fo  in  an  opposite  direction  during  the 
next  6  h.  The  two  lobes,  therefore,  seem  to  be  constantly 
closing  or  opening,  though  to  a  very  small  distance ;  for  we  must 
remember  that  the  little  triangle  of  paper  affixed  to  the  marginal 
spike  increased  its  length,  and  thus  exaggerated  somewhat  the 
movement.  Similar  observations,  with  the  important  difference 
that  the  petiole  was  left  free  and  the  plant  kept  under  a  high 
temperature,  were  made  on  a  leaf,  which  was  healthy,  but  so  old 
that  it  did  not  close  when  its  sensitive  hairs  were  repeatedly 
touched,  though  judging  from  other  cases  it  would  have  slowly 
closed  if  it  had  been  stimulated  by  animal  matter.  The  apex  of 
the  triangle  was  in  almost,  though  not  quite,  constant  movement, 
sometimes  in  one  direction  and  sometimes  in  an  opposite  one ; 
and  it  thrice  crossed  five  divisions  of  the  micrometer  (i.e.  y^  of 
an  inch)  in  30  m.  This  movement  on  so  small  a  scale  is  hardly 
comparable  with  ordinary  circumnutation ;  but  it  may  perhaps 
be  compared  with  the  zigzag  lines  and  little  loops,  by  which  the 
larger  ellipses  made  by  other  plants  are  often  interrupted. 

In  the  first  chapter  of  this  volume,  the  remarkable  oscillatory 
movements  of  the  circumnutating  hypocotyl  of  the  cabbage 
have  been  described.  The  leaves  of  Dionsea  present  the  same 
phenomenon,  which  is  a  wonderful  one,  as  viewed  under  a  low 
power  (2-inch  object-glass),  with  an  eye-piece  micrometer  of 
which  each  division  (-^  of  an  inch)  appeared  as  a  rather  wide 
space.  The  young  unexpanded  leaf,  of  which  the  circumnutating 
movements  were  traced  (Fig.  106),  had  a  glass  filament  fixed 
perpendicularly  to  it;  and  the  movement  of  the  apex  was 
observed  in  the  hot-house  (temp.  84°  to  86°  F.),  with  light 
admitted  only  from  above,  and  with  any  lateral  currents  of  air 


CHAP.  IV.  DICOTYLEDONS.  243 

excluded.  The  apex  sometimes  crossed  one  or  two  divisions  of 
the  micrometer  at  an  imperceptibly  slow  rate,  but  generally  it 
moved  onwards  by  rapid  starts  or  jerks  of  y^2^  or  yoVo*  and  in 
one  instance  of  y^^  of  an  inch.  After  each  jerk  forwards,  the 
apex  drew  itself  backwards  with  comparative  slowness  for  part 
of  the  distance  which  had  just  been  gained ;  and  then  after  a 
very  short  time  made  another  jerk  forwards.  Four  conspi- 
cuous jerks  forwards,  with  slower  retreats,  were  seen  on  one 
occasion  to  occur  in  exactly  one  minute,  besides  some  minor 
oscillations.  As  far  as  we  could  judge,  the  advancing  and 
retreating  lines  did  not  coincide,  and  if  so,  extremely  minute 
ellipses  were  each  time  described.  Sometimes  the  apex  remained 
quite  motionless  for  a  short  period.  Its  general  course  during 
the  several  hours  of  observation  was  in  two  opposite  directions, 
so  that  the  leaf  was  probably  circumnutating. 

An  older  leaf  with  the  lobes  fully  expanded,  and  which  was 
afterwards  proved  to  be  highly  sensitive  to  contact,  was  next 
observed  in  a  similar  manner,  except  that  the  plant  was  exposed 
to  a  lower  temperature  in  a  room.  The  apex  oscillated  forwards 
and  backwards  in  the  same  manner  as  before ;  but  the  jerks  for- 
ward were  less  in  extent,  viz.  about  y^  inch ;  and  there  were 
longer  motionless  periods.  As  it  appeared  possible  that  the 
movements  might  be  due  t4  currents  of  air,  a  wax  taper  was 
held  close  to  the  leaf  during  one  of  the  motionless  periods,  but 
no  oscillations  were  thus  caused.  After  10  m.,  however,  vigorous 
oscillations  commenced,  perhaps  owing  to  the  plant  having  been 
warmed  and  thus  stimulated.  The  candle  was  then  removed  and 
before  long  the  oscillations  ceased ;  nevertheless,  when  looked  at 
again  after  an  interval  of  1  h.  30  m.,  it  was  again  oscillating. 
The  plant  was  taken  back  into  the  hot-house,  and  on  the 
following  morning  was  seen  to  be  oscillating,  though  not  very 
vigorously.  Another  old  but  healthy  leaf,  which  was  not  in  the 
least  sensitive  to  a  touch,  was  likewise  observed  during  two 
days  in  the  hot-house,  and  the  attached  filament  made  many 
little  jerks  forwards  of  about  y-^  or  only  y^o  of  an  inch. 

Finally,  to  ascertain  whether  the  lobes  independently  of  the 
petiole  oscillated,  the  petiole  of  an  old  leaf  was  cemented  close 
to  the  blade  with  shellac  to  the  top  of  a  little  stick  driven  into 
the  soil.  But  before  this  was  done  the  leaf  was  observed,  and 
found  to  be  vigorously  oscillating  or  jerking ;  and  after  it  had 
been  cemented  to  the  stick,  the  oscillations  of  about  -r-^  of 
an  inch  still  continued.  On  the  following  day  a  little  infusion 


244 


CmCUMNUTATION   OF  LEAVES.        CHAP.  IV. 


of  raw  meat  was  placed  on  the  leaf,  which  caused  the  lobes  to 
close  together  very  slowly  in  the  course  of  two  days ;  and  the 
oscillations  continued  during  this  whole  time  and  for  the  next 
two  days.  After  nine  additional  days  the  leaf  began  to  open 
and  the  margins  were  a  little  everted,  and  now  the  apex  of  the 
glass  filament  remained  for  long  periods  motionless,  and  then 
moved  backwards  and  forwards  for  a  distance  of  about  y^o  of 
an  inch  slowly,  without  any  jerks.  Nevertheless,  after  warming 
the  leaf  with  a  taper  held  close  to  it,  the  jerking  movement 
recommenced. 

This  same  leaf  had  been  observed  2£  months  previously,  and 
was  then  found  to  be  oscillating  or  jerking.  We  may  therefore 
infer  that  this  kind  of  movement  goes  on  night  and  day  for  a 
very  long  period ;  and  it  is  common  to  young  unexpanded  leaves 
and  to  leaves  so  old  as  to  have  lost  their  sensitiveness  to  a 
touch,  but  which  were  still  capable  of  absorbing  nitrogenous 
matter.  The  phenomenon  when  well  displayed,  as  in  the  young 
leaf  just  described,  is  a  very  interesting  one.  It  often  brought 
before  our  minds  the  idea  of  effort,  or  of  a  small  animal 
struggling  to  escape  from  some  constraint. 

(16.)  Eucalyptus  resinifera  (Myrtacese,  Fam.  94). — A  young  leaf, 
two  inches  in  length  together  with 
the, petiole,  produced  by  a  lateral 
shoot  from  a  cut-down  tree,  was 
observed  in  the  usual  manner. 
The  blade  had  not  as  yet  as- 
sumed its  vertical  position.  On 
June  7th  only  a  few  observations 
were  made,  and  the  tracing  merely 
showed  that  the  leaf  had  moved 
three  times  upwards  and  three 
downwards.  On  the  following 
day  it  was  observed  more  fre- 
quently; and  two  tracings  were 
made  (see  A  and  B,  Fig.  108),  as 
a  single  one  would  have  been  too 
complicated.  The  apex  changed 
its  course  13  times  in  the  course 
of  16  h.,  chiefly  up  and  down,  but 
The  actual  amount  of  movement 


Fig.  108. 


Eucalyptus  resinifera  :  circum  nu- 
tation of  a  leaf,  traced,  A,  from 
6.40  A.M.  to  1  P.M.  June  8th  ; 
B,  from  1  P.M.  8th  to  8.30  A.M. 
9th.  Apex  of  leaf  14£  inches 
from  the  horizontal  glass,  so 
figures  considerably  magnified. 


with  some  lateral  movement, 
in  any  one  direction  was  small. 
(17.)  Dahlia  (garden  var.)  (Composite,  Fam.  122). — A  fine  young 


CHAP.  IV. 


DICOTYLEDONS. 


245 


leaf  51  inches  in  length,  produced  by  a  young  plant  2  feet  high, 
growing  vigorously  in  a  large  pot,  was  directed  at  an  angle  of 
about  45°  beneath  the  horizon.  On  June  18th  the  leaf  descended 
from  10  A.M.  till  11.35  A.M.  (see  Fig.  109);  it  then  ascended 
greatly  till  6  P.M.,  this  ascent  being  probably  due  to  the  light 

Fig.  109. 


Dahh'i :  circumnutation  of  leaf,  traced  from  10  A.M.  June  18th  to  8.10  A.M. 
20th,  but  with  a  break  of  1  h.  40  m.  on  the  morning  of  the  19th,  as, 
owing  to  the  glass  filament  pointing  too  much  to  one  side,  the  pot  had 
to  be  slightly  moved  ;  therefore  the  relative  position  of  the  two  tracings 
is  somewhat  arbitrary.  The  figure  here  given  is  reduced  to  one-fifth  of 
the  original  scale.  Apex  of  leaf  9  inches  from  the  glass  in  the  line 
of  its  inclination,  and  4^  in  a  horizontal  line. 

coming  only  from  above.  It  zigzagged  between  6  P.M.  and 
10.35  P.M.,  and  ascended  a  little  during  the  night.  It  should  be 
remarked  that  the  vertical  distances  in  the  lower  part  of  the 
diagram  are  much  exaggerated,  .as  the  leaf  was  at  first  deflected 
beneath  the  horizon,  and  after  it  had  sunk  downwards,  the 
filament  pointed  in  a  very  oblique  line  towards  the  glass.  Next 


246  CIRCUMNUTATION  OF  LEAVES.         CHAP.  IV. 

day  the  leaf  descended  from  8.20  A.M.  till  7.15  P.M.,  then  zigzagged 
and  ascended  greatly  during  the  night.  On  the  morning  of  the 
20th  the  leaf  was  probably  beginning  to  descend,  though  the 
short  line  in  the  diagram  is  horizontal.  The  actual  distances 
travelled  by  the  apex  of  the  leaf  were  considerable,  but  could 
not  be  calculated  with  safety.  From  the  course  pursued  on  the 
second  day,  when  the  plant  had  accommodated  itself  to  the  light 
from  above,  there  cannot  be  much  doubt  that  the  leaves  undergo 
a  daily  periodic  movement,  sinking  during  the  day  and  rising 
at  night. 

(18.)  Mutisia  clematis  (Composite). — The  leaves  terminate  in 
tendrils  and  circumnutate  like  those  of  other  tendril-bearers ; 
but  this  plant  is  here  mentioned,  on  account  of  an  erroneous 
statement  *  which  has  been  published,  namely,  that  the  leaves 
sink  at  night  and  rise  during  the  day.  The  leaves  which 
behaved  in  this  manner  had  been  kept  for  some  days  in  a 
northern  room  and  had  not  been  sufficiently  illuminated.  A 
plant  therefore  was  left  undisturbed  in  the  hot-house,  and  three 
leaves  had  their  angles  measured  at  noon  and  at  10  P.M.  All 
three  were  inclined  a  little  beneath  the  horizon  at  noon,  but  one 
stood  at  night  2°,  the  second  21°,  and  the  third  10°  higher  than 
in  the  middle  of  the  day ;  so  that  instead  of  sinking  they  rise 
a  little  at  night. 

(19.)  Cyclamen  Persicum  (Primulacese,  Fam.  135). — A  young 
leaf,  1*8  of  an  inch  in  length,  petiole  included,  produced  by  an 
old  root-stock,  was  observed  during  three  days  in  the  usual 
manner  (Fig.  110).  On  the  first  day  the  leaf  fell  more  than  after- 
wards, apparently  from  adjusting  itself  to  the  light  from  above. 
On  all  three  days  it  fell  from  the  early  morning  to  about  7  P.M., 
and  from  that  hour  rose  during  the  night,  the  course  being 
slightly  zigzag.  The  movement  therefore  is  strictly  periodic. 
It  should  be  noted  that  the  leaf  would  have  sunk  each  evening 
a  little  lower  down  than  it  did,  had  not  the  glass  filament  rested 
between  5  and  6  P.M.  on  the  rim  of  the  pot.  The  amount  of 
movement  was  considerable ;  for  if  we  assume  that  the  whole 
leaf  to  the  base  of  the  petiole  became  bent,  the  tracing  would 
be  magnified  rather  less  than  five  times,  and  this  would  give 
to  the  apex  a  rise  and  fall  of  half  an  inch,  with  some  lateral 
movement.  This  amount,  however,  would  not  attract  attention 
without  the  aid  of  a  tracing  or  measurement  of  some  kind. 


The  Movements  and  Habits  of  Climbing  Plants,'  1875,  p.  118. 


CHAP.  IV. 


DICOTYLEDONS. 


247 


(20.)  Allamanda  Scliottii  (Apocynese,  Fam.  144). — The  young 
leaves  of  this  shrub  are  elongated,  with  the  blade  bowed  so  much 

Fig.  110. 


Cyclamen  Persicum :  circumnutation  of  leaf,  traced  from  6.45  A.M.  June  2nd 
to  6.40  A.M.  5th.     Apex  of  leaf  7  inches  from  the  vertical  glass. 

downwards  as  almost  to  form  a  semicircle.  The  chord — that 
is,  a  line  drawn  from  the  apex  of  the  blade  to  the  base  of  the 
petiole — of  a  young  leaf.  4f  inches  in  length,  stood  at  2.50  P.M.  on 


248 


CIRCUMNUTATION   OF   LEAVES. 


CHAP.  IV 


Dec.  5tli  at  an  angle  of  13°  beneath  the  horizon,  but  by  9.30P.M. 

the  blade  had  straightened  itself 
Flgt  11]'  so  much,    which    implies   the 

raising  of  the  apex,  that  the 
chord  now  stood  at  37°  above  the 
horizon,  and  had  therefore  risen 
50°.  On  the  next  day  similar 
angular  measurements  of  the 
same  leaf  were  made;  and  at 
noon  the  chord  stood  36°  be- 
neath the  horizon,  and  9.30  P.M. 
3£°  above  it,  so  had  risen  39J°. 
The  chief  cause  of  the  rising 
movement  lies  in  the  straighten- 
ing of  the  blade,  but  the  short 
petiole  rises  between  4°  and  5°. 
On  the  third  night  the  chord 
stood  at  35°  above  the  horizon, 
and  if  the  leaf  occupied  the 
same  position  at  noon,  as  on 
the  previous  day,  it  had  risen 
71°.  With  older  leaves  no  such 
change  of  curvature  could  be 
detected.  The  plant  was  then 
brought  into  the  house  and 
kept  in  a  north-east  room,  but 
at  night  there  was  no  change 
in  the  curvature  of  the  young 
leaves;  so  that  previous  expo- 
sure to  a  strong  light  is  appa- 
rently requisite  for  the  periodi- 
cal change  of  curvature  in  the 
blade,  and  for  the  slight  rising 
of  the  petiole. 

(21.)  Wigandia  (Hydroleaceas, 
Fam.  149).— Professor  Pfeffer 
informs  us  that  the  leaves  of  this 
plant  rise  in  the  evening ;  but  as 
we  do  not  know  whether  or  not 
the  rising  is  great,  this  species 
ought  perhaps  to  be  classed 
amongst  sleeping  plants. 


Petunia  violacea  :  downward  move- 
ment and  circumnutation  of  a 
very  young  leaf,  traced  from  10 
A.M.  June  2nd  to  9.20  A.M.  June 
6th.  N.B. — At  6.40  A.M.  on  the 
5th  it  was  necessary  to  move  the 
pot  a  little,  and  a  new  tracing 
was  begun  at  the  point  where 
two  dots  are  not  joined  in  the 
diagram.  Apex  of  leaf  7  inches 
from  the  vertical  glass.  Temp, 
generally  17£°  C. 


CHAP.  IV.  DICOTYLEDONS.  249 

(22.)  Petunia  violaaa  (Solanese,  Fam.  157). — A  very  young 
leaf,  only  -|  inch  in  length,  highly  inclined  upwards,  was  observed 
for  four  days.  During  the  whole  of  this  time  it  bent  outwards 
and  downwards,  so  as  to  become  more  and  more  nearly  hori- 
zontal. The  strongly  marked  zigzag  line  in  the  figure  on  p.  248 
(Fig.  Ill),  shows  that  this  was  effected  by  modified  circum- 
nutation ;  and  during  the  latter  part  of  the  time  there  was  much 
ordinary  circumnutation  on  a  small  scale.  The  movement  in 
the  diagram  is  magnified  between  10  and  11  times.  It  exhibits 
a  clear  trace  of  periodicity,  as  the  leaf  rose  a  little  each  evening ; 
but  this  upward  tendency  appeared  to  be  almost  conquered  by 
the  leaf  striving  to  become  more  and  F-  112 

more  horizontal  as  it  grew  older.  The 
angles  which  two  older  leaves  formed 
together,  were  measured  in  the  even- 
ing and  about  noon  on  3  successive 
days,  and  each  night  the  angle  de- 
creased a  little,  though  irregularly. 

(23.)  Acanthus  mollis  (Acanthacese, 
Fam.  168).  —  The  younger  of  two 
leaves,  2}  inches  in  length,  petiole 
included,  produced  by  a  seedling 
plant,  was  observed  during  47  h. 
Early  on  each  of  the  three  morn- 
ings, the  apex  of  the  leaf  fell ;  and 
it  continued  to  fall  till  3  P.M.,  on 
the  two  afternoons  when  observed. 
After  3  P.M.  it  rose  considerably,  and 
continued  to  rise  on  the  second  night 
until  the  early  morning.  But  on 
the  first  night  it  fell  instead  of  rising, 
and  we  have  little  doubt  that  this  Acanthus  mo//ts:circumnuta- 
was  owing  to  the  leaf  being  very  tion  of  young  leaf,  traced 
young  and  becoming  through  epi-  % l™^^  ™ 
nastic  growth  more  and  more  hori-  of  leaf  n  inches  from  the 
zontal ;  for  it  may  be  seen  in  the  vertical  glass,  so  movement 
diagram  (Fig.  112),  that  the  leaf  stood  considerably  magnified. 
,  .  ,  ,,  ,.  ,  ,,  Figure  here  reduced  to  one- 

on  a  higher  level  on  the  first  than  on      haff    of    ori?inal     scale. 

the   second   day.     The  leaves  of  an      Temp.  15°-16£°  C. 
allied  species  (A.  spinosus)  certainly 

rose  every  night ;  and  the  rise  between  noon  and  10.15  P.M., 
when  measured  on  one  occasion,  was  10°.     This  rise  was  chiefly 


250  CIRCUMNUTATION   OF   LEAVES.         CHAP.  IV. 

or  exclusively  due  to  the  straightening  of  the  blade,  and  not  to 
the  movement  of  the  petiole.  We  may  therefore  conclude  that 
the  leaves  of  Acanthus  circumnutate  periodically,  falling  in  the 
morning  and  rising  in  the  afternoon  and  night. 

(24.)  Cannalis  sotiva  (Cannabinese,  Fam.  195). — We  have 
here  the  rare  case  of  leaves  moving  downwards  in  the  evening, 
but  not  to  a  sufficient  degree  to  be  called  sleep.*  In  the  early 
morning,  or  in  the  latter  part  of  the  night,  they  move  upwards. 
For  instance,  all  the  young  leaves  near  the  summits  of  several 
stems  stood  almost  horizontally  at  8  A.M.  May  29th,  and  at 
10.30  P.M.  were  considerably  declined.  On  a  subsequent  day  two 
leaves  stood  at  2  P.M.  at  21°  and  12°  beneath  the  horizon,  and  at 
10  P.M.  at  38°  beneath  it.  Two  other  leaves  on  a  younger  plant 
were  horizontal  at  2  P.M.,  and  at  10  P.M.  had  sunk  to  36°  beneath 
the  horizon.  With  respect  to  this  downward  movement  of  the 
leaves,  Kraus  believes  that  it  is  due  to  their  epinastic  growth. 
He  adds,  that  the  leaves  are  relaxed  during  the  day,  and  tense 
at  night,  both  in  sunny  and  rainy  weather. 

(25.)  Pinus  pinaster  (Coniferse,  Fam.  223). — The  leaves  on  the 
summits  of  the  terminal  shoots  stand  at  first  in  a  bundle  almost 
upright,  but  they  soon  diverge  and  ultimately  become  almost 
horizontal.  The  movements  of  a  young  leaf,  nearly  one  inch  in 
length,  on  the  summit  of  a  seedling  plant  only  3  inches  high, 
were  traced  from  the  early  morning  of  June  2nd  to  the  evening 
of  the  7th.  During  these  five  days  the  leaf  diverged,  and  its  apex 
descended  at  first  in  an  almost  straight  line ;  but  during  the  two 
latter  days  it  zigzagged  so  much  that  it  was  evidently  circumnu- 
tating.  The  same  little  plant,  when  grown  to  a  height  of  5  inches, 
was  again  observed  during  four  days.  A  filament  was  fixed 
transversely  to  the  apex  of  a  leaf,  one  inch  in  length,  and  which 
had  already  diverged  considerably  from  its  originally  upright 
position.  It  continued  to  diverge  (see  A,  Fig.  113),  and  to 
descend  from  11.45  A.M.  July  31st  to  6.40  A.M.  Aug.  1st.  On 
August  1st  it  circumnutated  about  the  same  small  space,  and 
again  descended  at  night.  Next  morning  the  pot  was  moved 
nearly  one  inch  to  the  right,  and  a  new  tracing  was  begun  (B). 
From  this  time,  viz.,  7  A.M.  August  2nd  to  8.20  A.M.  on  the  4th, 


*  We  were  led  to  observe  this  Flora,  1879,  p.  66.   We  regret  that 

plant  by  Dr.  Carl  Kraus'  paper,  we  cannot  fully  understand  parts 

*  Beitrage  zur  Kentniss  der  Bewe-  of  this  paper, 
gungen  Wachsonder  Laubblatter,' 


CHAP.  IV. 


DICOTYLEDONS. 


251 


the  leaf  manifestly  circumnutated.  It  does  not  appear  from  the 
diagram  that  the  leaves  move  periodically,  for  the  descending 
course  during  the  first  two  nights,  was  clearly  due  to  epinastic 


.  31s." 


Fig.  113. 


Pinus  pinaster:  circumnutation  of  young  leaf,  traced  from  11.45  A.M. 
July  JUst  to  8.20  A.M.  Aug.  4th.  At  7  A.M.  Aug.  2nd  the  pot  was 
moved  an  inch  to  one  side,  so  that  the  tracing  consists  of  two  figures. 
Apex  of  leaf  14^  inches  from  the  vertical  glass,  so  movements  much 
magnified. 

growth,  and  at  the  close  of  our  observations  the  leaf  was  not 
nearly  so  horizontal  as  it  would  ultimately  become. 

Pinus  austriaca. — Two  leaves,  3  inches  in  length,  but  not 


252 


CIKCUMNUTATIOX   OF   LEAVES.         CHAP.  IV. 


quite  fully  grown,  produced  by  a  lateral  shoot,  on  a  young  tree 
3  feet  in  height,  were  observed  during  29  h.  (July  31st),  in  the 
same  manner  as  the  leaves  of  the  previous  species.     Both  these 
leaves  certainly  circumnutated,  making 
Fig.  114.  within  the  above  period  two,  or  two  and 

a  half,  small,  irregular  ellipses. 

(26.)  Cycas  pectinata  (Cycadese,  Fam. 
224).  —  A  young  leaf,  Hi  inches  in 
length,  of  which  the  leaflets  had  only 
recently  become  uncurled,  was  observed 
during  47  h.  30  m.  The  main  petiole 
was  secured  to  a  stick  at  the  base  of  the 
two  tenninal  leaflets.  To  one  of  the 
latter,  3f  inches  in  length,  a  filament 
was  fixed ;  the  leaflet  was  much  bowed 
downward,  but  as  the  terminal  part  was 
upturned,  the  filament  projected  almost 
horizontally.  The  leaflet  moved  (see 
Fig.  114)  largely  and  periodically,  for  it 
fell  until  about  7  P.M.  and  rose  during 
the  night,  falling  again  next  morning 
after  6.40  A.M.  The  descending  lines 
are  in  a  marked  manner  zigzag,  and  so 
probably  would  have  been  the  ascending 
lines,  if  they  had  been  traced  throughout 
the  night. 


Cycas  pectinata :  circum- 
nutation  of  one  of  the 
terminal  leaflets,  traced 
from  8.30  A.M.  June 
22nd  to  8  A.M.  June 
24th.  Apex  of  leaflet 
7f  inches  from  the  ver- 
tical glass,  so  tracing 
not  greatly  magnified, 
and  here  reduced  to 
one-third  of  original 
scale ;  temp.  19°-2i°  C. 


ClRCUMNUTATION  OF  LEAVES: 
MONOCOTYLEDONS. 

(27.)  Canna  Warscewiczii  (Cannaceae, 
Fam.  2). — The  movements  of  a  young 
leaf,  8  inches  in  length  and  3£  in 
breadth,  produced  by  a  vigorous  young 
plant,  were  observed  during  45  h. 
50m.,  as  shown  in  Fig.  115.  The  pot 

was  slided  about  an  inch  to  the  right  on  the  morning  of  the 
llth,  as  a  single  figure  would  have  been  too  complicated ;  but 
the  two  figures  are  continuous  in  time.  The  movement  is 
periodical,  as  the  leaf  descended  from  the  early  morning  until 
about  5  P.M.,  and  ascended  during  the  rest  of  the  evening  and 


CHAP.  IV. 


MONOCOTYLEDONS. 


253 


part  of  the  night.    On  the  evening  of  the  llth  it  circumnutated 
on  a  small  scale  for  some  time  about  the  same  spot. 

Fig.  115. 


Canna  Warscewiczii :  circumnutation  of  leaf,  traced  (A)  from  11.30  A.M 
June  10th  to  6.40  A.M.  llth ;  and  (B)  from  6.40  A.M.  llth  to  8.40  A.M. 
12th.  Apex  of  leaf  9  inches  from  the  vertical  glass. 


Fig.  116. 


(28.)  Iris  pseudo-acorus  (Iridese,  Fam.  10). — The  movements 
of  a  young  leaf,  rising  13  inches  above  the  water  in  which  the 
plant  grew,  were  traced  as  shown  in  the 
figure  (Fig.  116),  during  27  h.  30  m. 
It  manifestly  circumnutated,  though 
only  to  a  small  extent.  On  the  second 
morning,  between  6.40  A.M.  and  2  P.M. 
(at  which  latter  hour  the  figure  here 
given  ends),  the  apex  changed  its  course 
five  times.  During  the  next  8  h.  40  m.  it 
zigzagged  much,  and  descended  as  far 
as  the  lowest  dot  in  the  figure,  making 
in  its  course  two  very  small  ellipses ; 
but  if  these  lines  had  been  added  to 
the  diagram  it  would  have  been  too 
complex. 

(29.)  Crinum  Capense  (Amaryllidese, 
Fam.  11).— The  leaves  of  this  plant 
are  remarkable  for  their  great  length 
and  narrowness:  one  was  measured 
and  found  to  be  53  inches  long  and 
only  1*4  broad  at  the  base.  Whilst  quite  young  they  stand  up 
almost  vertically  to  the  height  of  about  a  foot;  afterwards 


Iris  pseudo-acorus  :  circum- 
nutation of  leaf,  traced 
from  10.30  A.M.  May  28th 
to  2  P.M.  29th.  Tracing 
continued  to  11  P.M.,  but 
not  here  copied.  Apex 
of  leaf  12  inches  beneath 
the  horizontal  glass,  so 
figure  considerably  mag- 
nified. Temp.  15°-16°  C. 


254  CIRCUMNUTATION   OF   LEAVES.         CHAP.  IV. 

their  tips  begin  to  bend  over,  and  subsequenily  hang  vertically 
down,  and  thus  continue  to  grow.  A  rather  young  leaf  was 
selected,  of  which  the  dependent  tapering  point  was  as  yet  only 
5g  inches  in  length,  the  upright  basal  part  being  20  inches  high, 
though  this  part  would  ultimately  become  shorter  by  being 
more  bent  over.  A  large  bell-glass  was  placed  over  the  plant, 
with  a  black  dot  on  one  side ;  and  by  bringing  the  dependent 
apex  of  the  leaf  into  a  line  with  this  dot,  the  accompanying 
figure  (Fig.  117)  was  traced  on  the  other  side  of  the  bell,  during 
2£  days.  During  the  first  day  (22nd)  the  tip  travelled  laterally 
far  to  the  left,  perhaps  in  consequence  of  the  plant  having  been 

Fig.  117. 


Crinum  capense :  circumnutation  of  dependent  tip  of  young  leaf,  traced  on 
a  bell-glass,  from  10.30  P.M.  May  22nd  to  10.15  A.M.  25th.  Figure  not 
greatly  magnified. 

disturbed ;  and  the  last  dot  made  at  10.30  P.M.  on  this  day  is 
alone  here  given.  As  we  see  in  the  figure,  there  can  be  no 
doubt  that  the  apex  of  this  leaf  circumnutated. 

A  glass  filament  with  little  triangles  of  paper  was  at  the 
same  time  fixed  obliquely  across  the  tip  of  a  still  younger  leaf, 
which  stood  vertically  up  and  was  as  yet  straight.  Its  move- 
ments were  traced  from  3  P.M.  May  22nd  to  10.15  A.M.  25th. 
The  leaf  was  growing  rapidly,  so  that  the  apex  ascended  greatly 
during  this  period ;  as  it  zigzagged  much  it  was  clearly  circum- 
nutating,  and  it  apparently  tended  to  form  one  ellipse  each 
day.  The  lines  traced  during  the  night  were  much  more  vertical 
than  those  traced  during  the  day ;  and  this  indicates  that  the 
tracing  would  have  exhibited  a  nocturnal  rise  and  a  diurnal 
fall,  if  the  leaf  had  not  grown  so  quickly.  The  movement  of 
this  same  leaf  after  an  interval  of  six  days  (May  31st),  by  which 
time  the  tip  had  curved  outwards  into  a  horizontal  position, 


CHAP.  IV.  MONOCOTYLEDONS.  255 

and  had  thus  made  the  first  step  towards  becoming  dependent, 
was  traced  orthogonically  by  the  aid  of  a  cube  of  wood  (in  the 
manner  before  explained) ;  and  it  was  thus  ascertained  that  the 
actual  distance  travelled  by  the  apex,  and  due  to  circum nutation, 
was  3£  inches  in  the  course  of  20£  h.  During  the  next  24  h.  it 
travelled  2s  inches.  The  circumnutating  movement,  therefore, 
of  this  young  leaf  was  strongly  marked. 

(30.)  Pancratium  littorale  (Amaryllidese). — The  movements, 
much  magnified,  of  a  leaf,  9  inches  in  length  and  inclined  at 
about  45°  above  the  horizon,  were  traced  during  two  days.  On 
the  .first  day  it  changed  its  course  completely,  upwards  and 
downwards  and  laterally,  9  times  in  12  h. ;  and  the  figure  traced 
apparently  represented  five  ellipses.  On  the  second  day  it  was 
observed  seldomer,  and  was  therefore  not  seen  to  change  its 
course  so  often,  viz.,  only  6  times,  but  in  the  same  complex 
manner  as  before.  The  movements  were  small  in  extent,  but 
there  could  be  no  doubt  about  the  circumnutation  of  the  leaf. 

(31.)  Imatophyllum.  vel  Olivia  (sp.  ?)  (Amaryllidese). — A  long 
glass  filament  was  fixed  to  a  leaf,  and  the  angle  formed  by  it 
with  the  horizon  was  measured  occasionally  during  three  suc- 
cessive days.  It  fell  each  morning  until  between  3  and  4  P.M., 
and  rose  at  night  The  smallest  angle  at  any  time  above  the 
horizon  was  48°,  and  the  largest  50°;  so  that  it  rose  only  2° 
at  night;  but  as  this  was  observed  each  day,  and  as  similar 
observations  were  nightly  made  on  another  leaf  on  a  distinct 
plant,  there  can  be  no  doubt  that  the  leaves  move  periodically, 
though  to  a  very  small  extent.  The  position  of  the  apex  when 
it  stood  highest  was  '8  of  an  inch  above  its  lowest  point. 

(32.)  Pistia  stratiotes  (Aroidese,  Fam.  30).  —  Hofmeister 
remarks  that  the  leaves  of  this  floating  water-plant  are  more 
highly  inclined  at  night  than  by  day.*  We  therefore  fastened 
a  fine  glass  filament  to  the  midrib  of  a  moderately  young 
leaf,  and  on  Sept.  19th  measured  the  angle  which  it  formed 
with  the  horizon  14  times  between  9  A.M.  and  11.50  P.M.  The 
temperature  of  the  hot-house  varied  during  the  two  days  of 
observation  between  18i°  and  23£°  C.  At  9  A.M.  the  filament 
stood  at  32°  above  the  horizon ;  at  3.34  P.M.  at  10°  and  at 
11.50  P.M.  at  55°  ;  these  two  latter  angles  being  the  highest  and 
the  lowest  observed  during  the  day,  showing  a  difference  of  45°. 
The  rising  did  not  become  strongly  marked  until  between 


Die  Lehre  von  der  Pflanzenzelle,'  1867,  p.  327. 
12 


256  CIRCUMNUTATION   OF   LEAVES.         CHAP.  IV 

5  and  6  P.M.  On  the  next  day  the  leaf  stood  at  only  10°  above 
the  horizon  at  8.25  A.M.,  and  it  remained  at  about  15  3  till  past 
3  P.M.;  at  5.40  P.M.  it  was  23°,  and  at  9.30  P.M.  58°;  so  that 
the  rise  was  more  sudden  this  evening  than  on  the  previous 
one,  and  the  difference  in  the  angle  amounted  to  48°.  The 
movement  is  obviously  periodical,  and  as  the  leaf  stood  on  the 
first  night  at  55°,  and  on  the  second  night  at  58°  above  the 
horizon,  it  appeared  very  steeply  inclined.  This  case,  as  we 
shall  see  in  a  future  chapter,  ought  perhaps  to  have  been 
included  under  the  head  of  sleeping  plants. 

(33.)  Pontederia  (sp.  ?)  (from  the  highlands  of  St.  Catharina, 

Fig.  118. 


Pontederia  (sp.  ?)  :  circumnutation  of  leaf,  traced  from  4.50  P.M.  July  2nd 
to  10.15  A.M.  4th.  Apex  of  leaf  1G£  inches  from  the  vertical  glass,  so 
tracing  greatly  magnified.  Temp,  about  17°  C.,  and  therefore  rather 
too  low. 

Brazil)  (Pontederiaceae,  Fam.  46). — A  filament  was  fixed  across 
the  apex  of  a  moderately  young  leaf,  7s  inches  in  height,  and 
its  movements  were  traced  during  42 h  h.  (see  Fig.  118).  On 
the  first  evening,  when  the  tracing  was  begun,  and  during  ihe 
night,  the  leaf  descended  considerably.  On  the  next  morning 
it  ascended  in  a  strongly  marked  zigzag  line,  and  descended 
again  in  the  evening  and  during  the  night.  The  movement, 
therefore,  seems  to  be  periodic,  but  some  doubt  is  thrown  on 
this  conclusion,  because  another  leaf,  8  inches  in  height, 
appearing  older  and  standing  more  highly  inclined,  behaved 
differently.  During  the  first  ]2  h.  it  circumnutated  over  a 


CHAP.  IV.       CIKCUMNUTATION   OF   CRYPTOGAMS.        257 

small  space,  but  during  the  night  and  the  whole  following  day 
it  ascended  in  the  same  general  direction;  the  ascent  being 
effected  by  repeated  up  and  down  well-pronounced  oscillations. 


CKYPTOGAMS. 

(34.)  Nephrodium  molle  (Filices,  Fam.  1). — A  filament  was 
fixed  near  the  apex  of  a  young  frond  of  this  Fern,  17  inches 
in  height,  which  was  not  as  yet  fully  uncurled ;  and  its  move- 
ments were  traced  during  24  h.  We  see  in  Fig.  119  that  it 

Fig.  119. 


Nephrodium  mollc :  circumnutation  of  rachis,  traced  from  9.15  A.M.  May 
28th  to  9  A.M.  29th.     Figure  hero  given  two-thirds  of  original  scale. 

plainly  circumnutated.  The  movement  was  not  greatly  magnified 
as  the  frond  was  placed  near  to  the  vertical  glass,  and  would 
probably  have  been  greater  and  more  rapid  had  the  day  been 
warmer.  For  the  plant  was  brought  out  of  a  warm  greenhouse 
and  observed  under  a  skylight,  where  the  temperature  was 
between  15°  and  16°  C.  We  have  seen  in  Chap.  I.  that  a  frond  of 
this  Fern,  as  yet  only  slightly  lobed  and  with  a  rachis  only  *23 
inch  in  height,  plainly  circumnutated.* 


*  Mr.  Loomi-s  and  Prof,  Asa 
Gray  have  described  ('  Botanical 
Gazette,'  1880,  pp.  27,  43),  an 
extremely  curious  case  of  more- 
ment  in  the  fronds,  but  only  in 
the  fruiting  fronds,  of  Asplenium 
trichomanes.  They  move  almost 
as  rapidly  as  the  little  leaflets 


of  Desnmdium  gyrans,  alternately 
backwards  and  forwards  through 
from  20  1o  40  degrees,  in  a  plane  at 
right  angles  to  that  of  the  frond. 
The  apex  of  the  frond  describes  "  a 
long  and  very  narrow  ellipse,"  so 
that  it  circumnutates.  But  the 
movement  differs  from  ordinary 


258        CIRCUMNUTATION   OF   CRYPTOGAMS.       CHAP.  IY. 


Fig.  120. 


In  the  chapter  on  the  Sleep  of  Plants  the  conspicuous  circum- 
nutation  of  Marsilea  quadrijoliata  (Marsileacese,  Fam.  4)  will  be 
described. 

It  has  also  .been  shown  in  Chap.  I.  that  a  very  young  Sda- 
ginella  (Lycopodiaceaa,  Fam.  6),  only  4  inch  in  height,  plainly 
circumnutated ;  we  may  therefore  conclude  that  older  plants, 
whilst  growing,  would  do  the  same. 

(35.)  Lunularia  vulgaris  (Hepaticae,  Fam.  11,  Muscales).— 
The  earth  in  an  old  flower-pot  was 
coated  with  this  plant,  bearing 
gemmae.  A  highly  inclined  frond, 
which  projected  '3  inch  above  the 
soil  and  was  '4  inch  in  breadth,  was 
selected  for  observation.  A  glass 
hair  of  extreme  tenuity,  '75  inch 
in  length,  with  its  end  whitened, 
was  cemented  with  shellac  to  the 
frond  at  right  angles  to  its  breadth ; 
and  a  white  stick  with  a  minute 
black  spot  was  driven  into  the  soil 
close  behind  the  end  of  the  hair. 
The  white  end  could  be  accurately 
brought  into  a  line  with  the  black 
spot,  and  dots  could  thus  be  suc- 
cessively made  on  the  vertical 
glass-plate  in  front.  Any  move- 
ment of  the  frond  would  of  course 
be  exhibited  and  increased  by  the 
long  glass  hair ;  and  the  black  spot 
was  placed  so  close  behind  the  end 
of  the  hair,  relatively  to  the  dis- 
tance of  the  glass-plate  in  front, 
that  the  movement  of  the  end  was 
magnified  about  40  times.  Never- 
theless, we  are  convinced  that  our 
tracing  gives  a  fairly  faithful  re- 
presentation of  the  movements  of 

the  frond.     In  the  intervals  between  each  observation,  the  plant 
was  covered  by  a  small  bell-glass.     The  frond,  as  already  stated, 


Lunularia  vufgaris :  circumnutn- 
tion  of  a  frond,  traced  from 
9  A.M.  Oct  25th  to  8  A.M.  27th. 


circumnutation  as  it  occurs  only 
when  the  plant  is  exposed  to  the 
light;  even  artificial  light  "is 


sufficient   to  excite  motion  for  a 
few  minutes." 


CHAP.  IV.  CIKCUMNUTATION   OF   LEAVES.  259 

was  highly  inclined,  and  the  pot  stood  in  front  of  a  north-east 
window.  During  the  five  first  days  the  frond  moved  downwards 
or  became  less  inclined ;  and  the  long  line  which  was  traced 
was  strongly  zigzag,  with  loops  occasionally  formed  or  nearly 
formed;  and  this  indicated  circumnutation.  Whether  the  sink- 
ing was  due  to  epinastic  growth,  or  apheliotropism,  we  do  not 
know.  As  the  sinking  was  slight  on  the  fifth  day,  a  new  tracing 
was  begun  on  the  sixth  day  (Oct.  25th),  and  was  continued 
for  47  "h. ;  it  is  here  given  (Fig.  120).  Another  tracing  was  made 
on  the  next  day  (27th)  and  the  frond  was  found  to  be  still  cir- 
cumnutating,  for  during  14  h.  30  m.  it  changed  its  course  com- 
pletely (besides  minor  changes)  10  times.  It  was  casually 
observed  for  two  more  days,  and  was  seen  to  be  continually 
moving. 

The  lowest  members  of  the  vegetable  series,  the  Thallogens, 
apparently  circumuutate.  If  an  Oscillaria  be  watched  under 
the  microscope,  it  may  be  seen  to  describe  circles  about  every 
40  seconds.  After  it  has  bent  to  one  side,  the  tip  first  begins 
to  bend  back  to  the  opposite  side  and  then  the  whole  filament 
curves  over  in  the  same  direction.  Hofmeister*  has  given  a 
minute  account  of  the  curious,  but  less  regular  though  constant, 
movements  of  Spirogyra:  during  2.j  h.  the  filament  moved  4 
times  to  the  left  and  3  times  to  the  right,  and  he  refers  to  a 
movement  at  right  angles  to  the  above.  The  tip  moved  at  the 
rate  of  about  01  mm.  in  five  minutes.  He  compares  the  move- 
ment with  the  nutation  of  the  higher  plants,  f  We  shall  hereafter 
see  that  heliotropic  movements  result  from  modified  circurn- 
nutation,  and  as  unicellular  Moulds  bend  to  the  light  we  may 
infer  that  they  also  circumnutate. 

CONCLUDING  KEMAKKS  ON  THE  CIRCUMNUTATION 
OF  LEAVES. 

The  circumnutating  movements  of  young  leaves  in 
33  genera,  belonging  to  25  families,  widely  distributed 


*  « Ueber  die  Bewegungen  rler  1880,  vol.  iii.    p.  820)    that  the 

Faclen    der   Spirogyra    princess:  movements  of  Spirulina,  a  mem- 

Jahreshefte  des  Vereins  liir  vater-  ber  of  the  Oscillatorie<e,  are  closely 

liindische  Naturkunde  in   Wiirt-  analogous   "to   the    well-known 

lemberg,' 1874,  p.  211.  rotation   of  growing   shoots  and 

t  Zukal  also  remarks  (as  quoted  tendrils." 
in  'Journal  K.  Microscop.  Soo.,' 


260  CIRCUMNUTATION  OF  LEAVES.  CHAP.  IV. 

amongst  ordinary  and  gymnospermous  Dicotyledons 
and  amongst  Monocotyledons,  together  with  several 
Cryptogams,  have  now  been  described.  It  would, 
therefore,  not  be  rash  to  assume  that  the  growing 
leaves  of  all  plants  circumnutate,  as  we  Have  seen 
reason  to  conclude  is  the  case  with  cotyledons.  The 
seat  of  movement  generally  lies  in  the  petiole,  but 
sometimes  both  in  the  petiole  and  blade,  or  in  the 
blade  alone.  The  extent  of  the  movement  differed  much 
in  different  plants ;  but  the  distance  passed  over  was 
never  great,  except  with  Pistia,  which  ought  perhaps 
to  have  been  included  amongst  sleeping  plants.  The 
angular  movement  of  the  leaves  was  only  occasionally 
measured ;  it  commonly  varied  from  only  2°  (and  pro- 
bably even  less  in  some  instances)  to  about  10° ;  but 
it  amounted  to  23°  in  the  common  bean.  The  move- 
ment is  chiefly  in  a  vertical  plane,  but  as  the  ascending 
and  descending  lines  never  coincided,  there  was  always 
some  lateral  movement,  and  thus  irregular  ellipses 
were  formed.  The  movement,  therefore,  deserves  to 
be  called  one  of  circumnutation ;  for  all  circumnuta- 
ting  organs  tend  to  describe  ellipses, — that  is,  growth 
on  one  side  is  succeeded  by  growth  on  nearly  but  not 
quite  the  opposite  side.  The  ellipses,  or  the  zigzag 
lines  representing  drawn-out  ellipses,  are  generally 
very  narrow ;  yet  with  the  Camellia,  their  minor  axes 
were  half  as  long,  and  with  the  Eucalyptus  more  than 
half  as  long  as  their  major  axes.  In  the  case  of  Cissus, 
parts  of  the  figure  more  nearly  represented  circles  than 
ellipses.  The  amount  of  lateral  movement  is  therefore 
sometimes  considerable.  Moreover,  the  longer  axes 
of  the  successively  formed  ellipses  (as  with  the  Bean, 
Cissus,  and  Sea-kale),  and  in  several  instances  the 
zigzag  lines  representing  ellipses,  were  extended  in 
very  different  directions  during  the  same  day  or  on 


CHAP.  IV.          CIRCUMNUTATION   OP  LEAVES.  201 

the  next  day.  The  course  followed  was  curvilinear  or 
straight,  or  slightly  or  strongly  zigzag,  and  little  loops 
or  triangles  were  often  formed.  A  single  large  irregular 
ellipse  may  be  described  on  one  day,  and  two  smaller 
ones  by  the  same  plant  on  the  next  day.  With  Drosera 
two,  and  with  Lupinus,  Eucalyptus  and  Pancratium, 
several  were  formed  each  day. 

The  oscillatory  and  jerking  movements  of  the  leaves 
of  Dionsea,  which  resemble  those  of  the  hypocotyl  of 
the  cabbage,  are  highly  remarkable,  as  seen  under  the 
microscope.  They  continue  night  and  day  for  some 
months,  and  are  displayed  by  young  unexpanded  leaves, 
and  by  old  ones  which  have  lost  their  sensibility  to  a 
touch,  but  which,  after  absorbing  animal  matter,  close 
their  lobes.  We  shall  hereafter  meet  with  the  same 
kind  of  movement  in  the  joints  of  certain  Graminese, 
and  it  is  probably  common  to  many  plants  while  cir- 
cumnutating.  It  is,  therefore,  a  strange  fact  that  no 
such  movement  could  be  detected  in  the  tentacles  of 
Drosera  rotundifolia,  though  a  member  of  the  same 
family  with  Dionsea  ;  yet  the  tentacle  which  was  ob- 
served was  so  sensitive,  that  it  began  to  curl  inwards 
in  23  seconds  after  being  touched  by  a  bit  of  raw  meat. 

One  of  the  most  interesting  facts  with  respect  to 
the  circumnutation  of  leaves  is  the  periodicity  of  their 
movements ;  for  they  often,  or  even  generally,  rise  a 
little  in  the  evening  and  early  part  of  the  night,  and 
sink  again  on  the  following  morning.  Exactly  the 
same  phenomenon  was  observed  in  the  case  of  coty- 
ledons. The  leaves  in  16  genera  out  of  the  33  which 
were  observed  behaved  in  this  manner,  as  did  probably 
2  others.  Nor  must  it  be  supposed  that  in  the  remain- 
ing 15  genera  there  was  no  periodicity  in  their  move- 
ments ;  for  6  of  them  were  observed  during  too  short 
a  period  for  any  judgment  to  be  formed  on  this  head, 


262  CIRCUMNUTATION  OF  LEAVES.          CHAP.  IV. 

and  3  were  so  young  that  their  epinastic  growth, 
which  serves  to  bring  them  down  into  a  horizontal 
position,  overpowered  every  other  kind  of  movement. 
In  only  one  genus,  Cannabis,  did  the  leaves  sink  in 
the  evening,  and  Kraus  attributes  this  movement  to 
the  prepotency  of  their  epinastic  growth.  That  the 
periodicity  is  determined  by  the  daily  alternations 
of  light  and  darkness  there  can  hardly  be  a  doubt,  as 
will  hereafter  be  shown.  Insectivorous  plants  are 
very  little  affected,  as  far  as  their  movements  are  con- 
cerned, by  light ;  and  hence  probably  it  is  that  their 
leaves,  at  least  in  the  cases  of  Sarracenia,  Drosera,  and 
Dionaea,  do  not  move  periodically.  The  upward  move- 
ment in  the  evening  is  at  first  slow,  and  with  different 
plants  begins  at  very  different  hours ; — with  Glaucium 
as  early  as  11  A.M.,  commonly  between  3  and  5  P.M., 
but  sometimes  as  late  as  7  P.M.  It  should  be  observed 
that  none  of  the  leaves  described  in  this  chapter 
(except,  as  we  believe,  those  of  Lupinus  speciosus) 
possess  a  pulvinus;  for  the  periodical  movements  of 
leaves  thus  provided  have  generally  been  amplified 
into  so-called  s-leep-movements,  with  which  we  are  not 
here  concerned.  The  fact  of  leaves  and  cotyledons 
frequently,  or  even  generally,  rising  a  little  in  the 
evening  and  sinking  in  the  morning,  is  of  interest  as 
giving  the  foundation  from  which  the  specialised  sleep- 
movements  of  many  leaves  and  cotyledons,  not  pro- 
vided with  a  pulvinus,  have  been  developed.  The 
above  periodicity  should  be  kept  in  mind,  by  any  one 
considering  the  problem  of  the  horizontal  position  of 
leaves  and  cotyledons  during  the  day,  whilst  illumi- 
nated from  above. 


CHAP.  V.  MODIFIED   CIRCUMNUTATICXN. 


CHAPTER  V. 

MODIFIED  CIRCUMNUTATION  :    CLIMBING  PLANTS;   EPINASTIO  AND 
HYPONASTIC  MOVEMENTS. 

Circumnutation  modified  through  innate  causes  or  through  the  action 
of  external  'conditions — Innate  causes— Climbing  plants ;  similarity 
of  tiieir  movements  with  those  of  ordinary  plants;  increased  ampli- 
tude ;  occasional  points  of  difference— Epinastic  growth  of  young 
leaves — Hyponastic  growth  of  the  hypoeotyls  and  epicotyls  of  seed- 
lings— Hooked  tips  of  climbing  and  other  plants  due  to  modified 
circumnutation  —  Ampelopsis  tricuspidata  —  Smithia  Pfundii  — 
Straightening  of  the  tip  due  to  hyponasty — Epinastic  growth  and 
circumnutation  of  the  flower-peduncles  of  Trifolium  repens  and 
Oxalis  carnosa. 

THE  radicles,  hypocotyls  and  epicotyls  of  seedling 
plants,  even  before  they  emerge  from  the  ground,  and 
afterwards  the  cotyledons,  are  all  continually  circum- 
nutating.  So  it  is  with  the  stems,  stolons,  flower- 
peduncles,  and  leaves  of  older  plants.  We  may,  there- 
fore, infer  with  a  considerable  degree  of  safety  that  all 
the  growing  parts  of  all  plants  circumnutate.  Although 
this  movement,  in  its  ordinary  or  unmodified  state, 
appears  in  some  cases  to  be  of  service  to  plants, 
either  directly  or  indirectly — for  instance,  the  circum- 
nutation of  the  radicle  in  penetrating  the  ground,  or 
that  of  the  arched  hypocotyl  and  epicotyl  in  breaking 
through  the  surface — yet  circumnutation  is  so  general, 
or  rather  so  universal  a  phenomenon,  that  we  cannot 
suppose  it  to  have  been  gained  for  any  special  pur- 
pose. We  must  believe  that  it  follows  in  some  un- 
known way  from  the  manner  in  which  vegetable  tissues 
grow. 


264:  MODIFIED   CIRCUMNUTATION.  CHAP.  V. 

We  shall  now  consider  the  many  cases  in  which 
circumnutation  has  been  modified  for  various  special 
purposes ;  that  is,  a  movement  already  in  progress  is 
temporarily  increased  in  some  one  direction,  and  tem- 
porarily diminished  or  quite  arrested  in  other  direc- 
tions. These  cases  may  be  divided  in  two  sub-classes ; 
in  one  of  which  the  modification  depends  on  innate  or 
constitutional  causes,  and  is  independent  of  external 
conditions,  excepting  in  so  far  that  the  proper  ones  for 
growth  must  be  present.  In  the  second  sub-class  the 
modification  depends  to  a  large  extent  on  external 
agencies,  such  as  the  daily  alternations  of  light  and 
darkness,  or  light  alone,  temperature,  or  the  attraction 
of  gravity.  The  first  small  sub-class  will  be  considered 
in  the  present  chapter,  and  the  second  sub-class  in  the 
remainder  of  this  volume. 

THE  CIRCUMNUTATION  OF  CLIMBING  PLANTS. 

The  simplest  case  of  modified  circumnutation  is  that 
offered  by  climbing  plants,  with  the  exception  of 
those  which  climb  by  the  aid  of  motionless  hooks  or 
of  rootlets  ;  for  the  modification  consists  chiefly  in  the 
greatly  increased  amplitude  of  the  movement.  This 
would  follow  either  from  greatly  increased  growth  over 
a  small  length,  or  more  probably  from  moderately  in- 
creased growth  spread  over  a  considerable  length  of  the 
moving  organ,  preceded  by  turgescence,  and  acting  suc- 
cessively on  all  sides.  The  circumnutation  of  climbers 
is  more  regular  than  that  of  ordinary  plants ;  but  in 
almost  every  other  respect  there  is  a  close  similarity 
between  their  movements,  namely,  in  their  tendency 
to  describe  ellipses  directed  successively  to  all  points 
of  the  compass — in  their  courses  being  often  inter- 
rupted by  zigzag  lines,  triangles,  loops,  or  small 


CHAP,  V.  CLIMBING  PLANTS.  265 

ellipses — in  the  rate  of  movement,  and  in  different 
species  revolving  once  or  several  times  within  the  same 
length  of  time.  In  the  same  internode,  the  move- 
ments cease  first  in  the  lower  part  and  then  slowly 
upwards.  In  both  sets  of  cases  the  movement  may  be 
modified  in  a  closely  analogous  manner  by  geotropism 
and  by  heliotropism ;  though  few  climbing  plants  are 
heliotropic.  Other  points  of  similarity  might  be 
pointed  out. 

That  the  movements  of  climbing  plants  consist  of 
ordinary  circumnutation,  modified  by  being  increased 
in  amplitude,  is  well  exhibited  whilst  the  plants  are 
very  young ;  for  at  this  early  age  they  move  like  other 
seedlings,  but  as  they  grow  older  their  movements 
gradually  increase  without  undergoing  any  other 
change.  That  this  power  is  innate,  and  is  not  excited 
by  any  external  agencies,  beyond  those  necessary  for 
growth  and  vigour,  is  obvious.  No  one  doubts  that 
this  power  has  been  gained  for  the  sake  of  enabling 
climbing  plants  to  ascend  to  a  height,  and  thus  to 
reach  the  light.  This  is  effected  by  two  very  different 
methods;  first,  by  twining  spirally  round  a  support, 
but  to  do  so  their  stems  must  be  long  and  flexible  ; 
and,  secondly,  in  the  case  of  leaf-climbers  and  tendril- 
bearers,  by  bringing  these  organs  into  contact  with  a 
support,  which  is  then  seized  by  the  aid  of  their 
sensitiveness.  It  may  be  here  remarked  that  these 
latter  movements  have  no  relation,  as  far  as  we  can 
judge,  with  circumnutation.  In  other  cases  the  tips 
of  tendrils,  after  having  been  brought  into  contact  with 
a  support,  become  developed  into  little  discs  which 
adhere  firmly  to  it. 

We  have  said  that  the  circumnutation  of  climbing 
plants  differs  from  that  of  ordinary  plants  chiefly  by 
its  greater  amplitude.  But  most  leaves  circumnutate 


266  MODIFIED  CIRCUMNUTATION.  CHAP.  V. 

in  an  almost  vertical  plane,  and  therefore  describe  very 
narrow  ellipses,  whereas  the  many  kinds  of  tendrils 
which  consist  of  metamorphosed  leaves,  make  much 
broader  ellipses  or  nearly  circular  figures ;  and  thus 
they  have  a  far  better  chance  of  catching  hold  of  a 
support  on  any  side.  The  movements  of  climbing 
plants  have  also  been  modified  in  some  few  other 
special  ways.  Thus  the  circumnutating  stems  of  Sol- 
nanum  dulcamara  can  twine  round  a  support  only 
when  this  is  as  thin  and  flexible  as  a  string  or  thread. 
The  twining  stems  of  several  British  plants  cannot 
twine  round  a  support  when  it  is  more  than  a  few 
inches  in  thickness ;  whilst  in  tropical  forests  some 
can  embrace  thick  trunks  ;*  and  this  great  difference 
in  power  depends  on  some  unknown  difference  in 
their  manner  of  circumnutation.  The  most  remarkable 
special  modification  of  this  movement  which  we  have 
observed  is  in  the  tendrils  of  Ecliinocystis  lobata  ;  these 
are  usually  inclined  at  about  45°  above  the  horizon, 
but  they  stiffen  and  straighten  themselves  so  as  to 
stand  upright  in  a  part  of  their  circular  course,  namely, 
when  they  approach  and  have  to  pass  over  the  s.ummit 
of  the  shoot  from  which  they  arise.  If  they  had  not 
possessed  and  exercised  this  curious  power,  they  would 
infallibly  have  struck  against  the  summit  of  the  shoot 
and  been  arrested  in  their  course.  As  soon  as  one  of 
these  tendrils  with  its  three  branches  begins  to  stiffen 
itself  and  rise  up  vertically,  the  revolving  motion 
becomes  more  rapid ;  and  as  soon  as  it  has  passed 
over  the  point  of  difficulty,  its  motion  coinciding 
with  that  from  its  own  weight,  causes  it  to  fall  into  its 
previously  inclined  position  so  quickly,  that  the  apex 
can  be  seen  travelling  like  the  hand  of  a  gigantic  clock 


*  'The  Movements  and  Habits  of  Climbing  Plants,'  p.  36. 


CHAP.  V.  EPINASTY  AND  HYPONASTY.  267 

A  large  number  of  ordinary  leaves  and  leaflets  and 
a  few  flower-peduncles  are  provided  with  pulvini ;  but 
this  is  not  the  case  with  a  single  tendril  at  present 
known.  The  cause  of  this  difference  probably  lies  in 
the  fact,  that  the  chief  service  of  a  pulvinus  is  to 
prolong  the  movement  of  the  part  thus  provided  after 
growth  has  ceased ;  and  as  tendrils  or  other  climbing- 
organs  are  of  use  only  whilst  the  plant  is  increasing 
in  height  or  growing,  a  pulvinus  which  served  to 
prolong  their  movements  would  be  useless. 

It  was  shown  in  the  last  chapter  that  the  stolons  or 
runners  of  certain  plants  circumnutate  largely,  and 
that  this  movement  apparently  aids  them  in  finding  a 
passage  between  the  crowded  stems  of  adjoining  plants. 
If  it  could  be  proved  that  their  movements  had  been 
modified  and  increased  for  this  special  purpose,  they 
ought  to  have  been  included  in  the  present  chapter ; 
but  as  the  amplitude  of  their  revolutions  is  not  so 
conspicuously  different  from  that  of  ordinary  plants, 
as  in  the  case  of  climbers,  we  have  no  evidence  on 
this  head.  We  encounter  the  same  doubt  in  the  case 
of  some  plants  which  bury  their  pods  in  the  ground. 
This  burying  process  is  certainly  favoured  by  the 
circumnutation  of  the  flower-peduncle ;  but  we  do  not 
know  whether  it  has  been  increased  for  this  special 
purpose. 

EPINASTY— HYPONASTY. 

The  term  epinasty  is  used  by  De  Tries  *  to  express 
greater  longitudinal  growth  along  the  upper  than 


*  'Arbciten    des    Bot.     Inst.,  two  terms  as  first  used  by  Schim- 

in  Wiirzburg/  Heftii.  1872,  p.  223.  per,  and  tLey  have  been  adopttd 

De  Vries   has    slightly  modified  in  this  sense  by  Sachs. 
(p.  252}  the  meaning  of  the  above 


268  MODIFIED  C1ECUMNUTATIOX.  CHAP.  V. 

along  the  lower  side  of  a  part,  which  is  thus  caused  to 
bend  downwards ;  and  hyponasty  is  used  for  the  reversed 
process,  by  which  the  part  is  made  to  bend  upwards. 
These  actions  come  into  play  so  frequently  that  the 
use  of  the  above  two  terms  is  highly  convenient.  The 
movements  thus  induced  result  from  a  modified  form 
of  circumnutation ;  for,  as  we  shall  immediately  see, 
an  organ  under  the  influence  of  epinasty  does  not 
generally  move  in  a  straight  line  downwards,  or  under 
that  of  hyponasty  upwards,  but  oscillates  up  and  down 
with  some  lateral  movement :  it  moves,  however,  in  a 
preponderant  manner  in  one  direction.  This  shows 
that  there  is  some  growth  on  all  sides  of  the  part,  but 
more  on  the  upper  side  in  the  case  of  epinasty,  and 
more  on  the  lower  side  in  that  of  hyponasty,  than  on 
the  other  sides.  At  the  same  time  there  may  be  in 
addition,  as  De  Yries  insists,  increased  growth  on  one 
side  due  to  geotropism,  and  on  another  side  due  to 
heliotropism ;  and  thus  the  effects  of  epinasty  or  of 
hyponasty  may  be  either  increased  or  lessened. 

He  who  likes,  may  speak  of  ordinary  circumnutation 
as  being  combined  with  epinasty,  hyponasty,  the  effects 
of  gravitation,  light,  &c. ;  but  it  seems  to  us,  from 
reasons  hereafter  to  be  given,  to  be  more  correct  to 
say  that  circumnutation  is  modified  by  these  several 
agencies.  We  will  therefore  speak  of  circumnutation, 
which  is  always  in  progress,  as  modified  by  epinasty, 
hyponasty,  geotropism,  or  other  agencies,  whether 
internal  or  external. 

One  of  the  commonest  and  simplest  cases  of  epinasty  is  that 
offered  by  leaves,  which  at  an  early  age  are  crowded  together 
round  the  buds,  and  diverge  as  they  grow  older.  Sachs  first 
remarked  that  this  was  due  to  increased  growth  along  the  upper 
side  of  the  petiole  and  blade ;  and  De  Vries  has  now  shown  in 
more  detail  that  the  movement  is  thus  caused,  aided  slightly  by 


CHAP.  V.  EPINASTY   AND  HYPONASTY.  269 

the  weight  of  the  leaf,  and  resisted  as  he  believes  by  apogeo- 
tropism,  at  least  after  the  leaf  has  somewhat  diverged.  In  our 
observations  on  the  circumnutation  of  leaves,  some  were  selected 
which  were  rather  too  young,  so  that  they  continued  to  diverge 
or  sink  downwards  whilst  their  movements  were  being  traced. 
This  may  be  seen  in  the  diagrams  (Figs.  98  and  112,  pp.  232 
and  249)  representing  the  circumnutation  of  the  young  leaves  of 
Acanthus  mollis  and  Pelargonium  zonale.  Similar  cases  were  ob- 
served with  Drosera.  The  movements  of  a  young  leaf,  only  I  inch 
in  length,  of  Petunia  violacea  were  traced  during  four  days,  and 
offers  a  better  instance  (Fig.  Ill,  p.  248),  as  it  diverged  during 
the  whole  of  this  time  in  a  curiously  zigzag  line  with  some  of  the 
angles  sharply  acute,  and  during  the  latter  days  plainly  circum- 
nutated.  Some  young  leaves  of  about  the  same  age  on  a  plant 
of  this  Petunia,  which  had  been  laid  horizontally,  and  on  another 
plant  which  was  left  upright,  both  being  kept  in  complete  dark- 
ness, diverged  in  the  same  manner  for  48  h.,  and  apparently 
were  not  aifected  by  apogeotropism ;  though  their  stems  were  in 
a  state  of  high  tension,  for  when  freed  from  the  sticks  to  which 
they  had  been  tied,  they  instantly  curled  upwards. 

The  leaves,  whilst  very  young,  on  the  leading  shoots  of  the 
Carnation  (Dianthus  caryophyllus)  are  highly  inclined  or  vertical ; 
and  if  the  plant  is  growing  vigorously  they  diverge  so  quickly 
that  they  become  almost  horizontal  in  a  day.  But  they  move 
downwards  in  a  rather  oblique  line  and  continue  for  some  time 
afterwards  to  move  in  the  same  direction,  in  connection,  we  pre- 
sume, with  their  spiral  arrangement  on  the  stem.  The  course 
pursued  by  a  young  leaf  whilst  thus  obliquely  descending  was 
traced,  and  the  line  was  distinctly  yet  not  strongly  zigzag ;  the 
larger  angles  formed  by  the  successive  lines  amounting  only  to 
135°,  154°,  and  163°.  The  subsequent  lateral  movement  (shown 
in  Fig.  96,  p.  231)  was  strongly  zigzag  with  occasional  circum- 
nutations.  The  divergence  and  sinking  of  the  young  leaves 
of  this  plant  seem  to  be  very  little  affected  by  geotropism  or 
heliotropism ;  for  a  plant,  the  leaves  of  which  were  growing 
rather  slowly  {as  ascertained  by  measurement)  was  laid  hori- 
zontally, and  the  opposite  young  leaves  diverged  from  one 
another  symmetrically  in  the  usual  manner,  without  any  up- 
turning in  the  direction  of  gravitation  or  towards  the  light. 

The  needle-like  leaves  of  Pinus  pinaster  form  a  bundle  whilst 
young ;  afterwards  they  slowly  diverge,  so  that  those  on  the  up- 
right shoots  become  horizontal.  The  movements  of  one  such 


270 


MODIFIED   CIRCUMXUTATION. 


CHAP.  V, 


Fig.  121. 


young  leaf  was  traced  during  4£  days,  and  the  tracing  here  given 
(Fig.  121)  shows  that  it  descended  at  first  in  a  nearly  straight 

line,  but  afterwards  zigzagged, 
making  one  or  two  little  loops. 
The  diverging  and  descend- 
ing movements  of  a  rather 
older  leaf  were  also  traced 
(see  former  Fig.  113,  p.  251) : 
it  descended  during  the  first 
day  and  night  in  a  some- 
what zigzag  line ;  it  then  cir- 
cumnutated  round  a  small 
space  and  again  descended. 
By  this  time  the  leaf  had 
nearly  assumed  its  final  posi- 
tion, and  now  plainly  circum- 
nutated.  As  in  the  case  of  the 
Carnation,  the  leaves,  whilst 
very  young,  do  not  seem  to  be 
much  affected  by  geotropism 
or  heliotropism,  for  those  on  a 
young  plant  laid  horizontally, 
and  those  on  another  plant 
left  upright.,  both  kept  in  the 
dark,  continued  to  diverge  in 
the  usual  manner  without 
bending  to  either  side. 

With  Cobcea  scandens,  the 
young  leaves,  as  they  succes- 
sively diverge  from  the  lead- 
ing shoot  which  is  bent  to 
one  side,  rise  up  so  as  to  pro- 
ject vertically,  and  they  retain 
this  position  for  some  time 
whilst  the  tendril  is  revolving. 
The  diverging  and  ascending 
movements  of  the  petiole  of 
one  such  a  leaf,  were  traced  on 
a  vertical  glass  under  a  sky- 
light ;  and  the  course  pursued 
was  in  most  parts  nearly 


Pintis  pinaster :  epinastic  downward 
movement  of  a  young  leaf,  pro- 
duced by  a  young  plant  in  a  pot, 
traced  on  a  vertical  glass  under  a 
sfcyiight,  from  6.45  A.M.  June  2nd 
to  10.40  P.M.  6th. 


straight,  but  there  were  two 


CHAP.  V.  EPINASTY  AND  HYPONASTY.  271 

well-marked  zigzags  (one  of  them  forming  an  angle  of  112°), 
and  this  indicates  circumnutation. 

The  still  closed  lobes  of  a  young  leaf  of  Dionaea  projected .  at 
right  angles  to  the  petiole,  and  were  in  the  act  of  slowly  rising. 
A  glass  filament  was  attached  to  the  under  side  of  the  midrib, 
and  its  movements  were  traced  on  a  vertical  glass.  It  circum- 
nutated  once  in  the  evening,  and  on  the  next  day  rose,  as  already 
described  (see  Fig.  106,  p.  240),  by  a  number  of  acutely  zigzag 
lines,  closely  approaching  in  character  to  ellipses.  This  move- 
ment no  doubt  was  due  to  epinasty,  aided  by  apogeotropism, 
for  the  closed  lobes  of  a  very  young  leaf  on  a  plant  which  had 
been  placed  horizontally,  moved  into  nearly  the  same  line  with 
the  petiole,  as  if  the  plant  had  stood  upright ;  but  at  the  same 
time  the  lobes  curved  laterally  upwards,  and  thus  occupied  an 
unnatural  position,  obliquely  to  the  plane  of  the  foliaceous  petiole. 

As  the  hypocotyls  and  epicotyls  of  some  plants  protrude  from 
the  seed-coats  in  an  arched  form,  it  is  doubtful  whether  the 
arching  of  these  parts,  which  is  invariably  present  when  they 
break  through  the  ground,  ought  always  to  be  attributed  to 
epinasty;  but  when  they  are  at  first  straight  and  afterwards 
become  arched,  as  often  happens,  the  arching  is  certainly  due  to 
epinasty.  As  long  as  the  arch  is  surrounded  by  compact  earth 
it  must  retain  its  form;  but  as  soon  as  it  rises  above  the 
surface,  or  even  before  this  period  if  artificially  freed  from  the 
surrounding  pressure,  it  begins  to  straighten  itself,  and  this  no 
doubt  is  mainly  due  to  hyponasty.  The  movement  of  the 
upper  and  lower  half  of  the  arch,  and  of  the  crown,  was  occa- 
sionally traced ;  and  the  course  was  more  or  less  zigzag,  showing 
modified  circumnutation. 

With  not  a  few  plants,  especially  climbers,  the  summit  of  the 
shoot  is  hooked,  so  that  the  apex  points  vertically  downwards. 
In  seven  genera  of  twining  plants  *  the  hooking,  or  as  it  has  been 
.called  by  Sachs,  the  nutation  of  the  tip,  is  mainly  due  to  an 
exaggerated  form  of  circumnutation.  That  is,  the  growth  is  so 
great  along  one  side  that  it  bends  the  shoot  completely  over  to 
the  opposite  side,  thus  forming  a  hook ;  the  longitudinal  line  or 
zone  of  growth  then  travels  a  little  laterally  round  the  shoot, 
and  the  hook  points  in  a  slightly  different  direction,  and  so 
onwards  until  the  hook  is  completely  reversed.  Ultimately  it 


*  '  The  Movements  and  Habits  of  Climbing  Plants,'  2nd  edit.  p.  13. 


272  MODIFIED   CIRCUMXUTATION.  CHAP.  V. 

comes  back  to  the  point  wlience  it  started.  This  was  ascertained 
by  painting  narrow  lines  with  Indian  ink  along  the  convex 
surface  of  several  hooks,  and  the  line  was  found  slowly  to  be- 
come at  first  lateral,  then  to  appear  along  the  concave  surface, 
and  ultimately  back  again  on  the  convex  surface.  In  the  case  of 
Lonicera  brachypoda  the  hooked  terminal  part  of  the  revolving 
shoot  straightens  itself  periodically,  but  is  never  reversed ;  that 
is,  the  periodically  increased  growth  of  the  concave  side  of  the 
hook  is  sufficient  only  to  straighten  it,  and  not  to  bend  it  over 
to  the  opposite  side.  The  hooking  of  the  tip  is  of  service  to 
twining  plants  by  aiding  them  to  catch  hold  of  a  support,  and 
afterwards  by  enabling  this  part  to  embrace  the  support  much 
more  closely  than  it  could  otherwise  have  done  at  lirst,  thus 
preventing  it,  as  we  often  observed,  from  being  blown  away  by  a 
strong  wind.  Whether  the  advantage  thus  gained  by  twining 
plants  accounts  for  their  summits  being  so  frequently  hooked, 
we  do  not  know,  as  this  structure  is  not  very  rare  with  plants 
which  do  not  climb,  and  with  some  climbers  (for  instance,  Vitis, 
Ampelopsis,  Cissus,  &c.)  to  whom  it  does  not  afford  any  assist^ 
ance  in  climbing. 

With  respect  to  those  cases  in  which  the  tip  remains  always 
bent  or  hooked  towards  the  same  side,  as  in  the  genera  just 
named,  the  most  obvious  explanation  is  that  the  bending  is  due 
to  continued  growth  in  excess  along  the  convex  side.  Wiesner, 
however,  maintains  *  that  in  all  cases  the  hooking  of  the  tip  is 
the  result  of  its  plasticity  and  weight, — a  conclusion  which  from 
what  we  have  already  seen  with  several  climbing  plants  is 
certainly  erroneous.  Nevertheless,  we  fully  admit  that  the 
weight  of  the  part,  as  well  as  geotropism,  &c.,  sometimes  come 
into  play. 

Ampelopsis  tricu*pidata. — This  plant  climbs  by  the  aid  of 
adhesive  tendrils,  and  the  hooked  tips  of  the  shoots  do  not 
appear  to  be  of  any  service  to  it.  The  hooking  depends  chiefly, 
as  far  as  we  could  ascertain,  on  the  tip  being  affected  by  epinasty 
and  geotropism ;  the  lower  and  older  parts  continually  straight- 
ening themselves  through  hyponasty  and  apogeotropism.  We 
believe  that  the  weight  of  the  apex  is  an  unimportant  element, 
because  on  horizontal  or  inclined  shoots  the  hook  is  often 
extended  horizontally  or  even  faces  upwards.  Moreover  shoots 
frequently  form  loops  instead  of  hooks;  and  in  this  case  the 


*   *  Sitzb.  der  k.  Akad.  der  Wissensch.,'  Vienna,  Jaa  1880,  p.  1G. 


CHAP.  V. 


EFINASTY  AND   HYPONASTY. 


273 


extreme  part,  instead  of  hang- 
ing vertically  down  as  would 
follow  if  weight  was  the  efficient 
cause,  extends  horizontally  or 
even  points  upwards.  A  shoot, 
which  terminated  in  a  rather 
open  hook,  was  fastened  in 
a  highly  inclined  downward 
position,  so  that  the  concave 
side  faced  upwards,  and  the 
result  was  that  the  apex  at  first 
curved  upwards.  This  ap- 
parently was  due  to  epinasty 
and  not  to  apogeotropism,  for 
the  apex,  soon  after  passing 
the  perpendicular,  curved  so 
rapidly  downwards  that  we 
could  not  doubt  that  the  move- 
ment was  at  least  aided  by 
geotropism.  In  the  course  of 
a  few  hours  the  hook  was  thus 
converted  into  a  loop  with  the 
apex  of  the  shoot  pointing 
straight  downwards.  The 
longer  axis  of  the  loop  was  at 
first  horizontal,  but  after- 
wards became  vertical.  During 
this  same  time  the  basal  part 
of  the  hook  (and  subsequently 
of  the  loop)  curved  itself  slowly 
upwards ;  and  this  must  have 
been  wholly  duo  to  apogeo- 
tropism in  opposition  to  hypo- 
nasty.  The  loop  was  then 
fastened  upside  down,  so  that 
its  basal  half  would  be  simul- 
taneously acted  on  by  hypo- 
nasty  (if  present)  and  by  apo- 
geotropism ;  and  now  it  curved 
itself  so  greatly  upwards  in 
the  course  of  only  4h.  that 
there  could  hardly  be  a  doubt 
that  both  forces  were  acting 


Fig-  122- 


9'JS'cim. 


Ampelopsis  tricuspidata ;  hyponastic 
movement  of  hooked  tip  of  leading 
shoot,  traced  from  8.10  A.M.  July 
13th  to  8  A.M.  15th.  Apex  of  shoot 
5g  inches  from  the  vertical  glass. 
Plant  illuminated  through  a  sky- 
light. Temp.  17t°-19°C.  Diagram 
reduced  to  one-third  of  original  scale. 


274 


MODIFIED   CIRCUMNUTATION. 


CHAP.  V. 


Kg.  123. 


Smithia  Pfundii :  hypoiiastic  movement 
of  thecurved  summitof  astern,  whilst 
straightening  itself,  traced  from  9 
A.M.  July  10th  to  3  P.M.  13th.  Apex 
9£  inches  from  the  vertical  glass. 
Diagram  reduced  to  one-fifth  of 
original  scale.  Plant  illuminated 
through  skylight ;  temp.  17£°-19°  C. 


together.  At  the  same  time 
the  loop  became  open  and 
was  thus  reconverted  into  a 
hook,  and  this  apparently 
was  effected  by  the  geotropic 
movement  of  the  apex  in 
opposition  to  epinasty.  In 
the  case  of  Ampelopsis  liede- 
racea,  weight  plays,  as  far  as 
we  could  judge,  a  more  im- 
portant part  in  the  hooking 
of  the  tip. 

In  order  to  ascertain 
whether  the  shoots  of  A.  tri- 
cuspidata  in  straightening 
themselves  under  the  com- 
bined action  of  hyponasty  and 
apogeotropism  moved  in  a 
simple  straight  course,  or 
whether  they  circumnutated, 
glass  filaments  were  fixed  to 
the  crowns  of  four  hooked 
tips  stand! i -g  in  their  natural 
position  ;  and  the  movements 
of  the  filaments  were  traced 
on  a  vertical  glass.  All  four 
tracings  resembled  each  other 
in  a  general  manner  ;  but  we 
will  give  only  one  (see  Fig. 
122,  p.  273).  Th3  filament 
rose  at  first,  which  shows 
that  the  hook  was  straighten- 
ing itself;  it  then  zigzagged, 
moving  a  little  to  the  left 
between  9.25  A.M.  and  9  P.M. 
From  this  latter  hour  on  the 
13th  to  10.50  A.M.  on  the  fol- 
lowing morning  (14th)  the 
hook  continued  to  straighten 
itself,  and  then  zigzagged  a 
short  distance  to  the  right. 
But  from  1  P.M.  to  10.40  P.M. 
on  the  14th  the  movement 


CHAI-.  V.  EPINASTY  AND   HYPONASTY.  275 

was  reversed  and  the  shoot  became  more  hooked.  During 
the  night,  after  10.40  P.M.  to  8.15  A.M.  on  the  15th,  the  hook 
again  opened  or  straightened  itself.  By  this  time  the  glass 
filament  had  become  so  highly  inclined  that  its  movements  could 
no  longer  be  traced  with  accuracy ;  and  by  1.30  P.M.  on  this  same 
day,  the  crown  of  the  former  arch  or  hook  had  become  perfectly 
straight  and  vertical.  There  can  therefore  be  no  doubt  that  the 
straightening  of  the  hooked  shoot  of  this  plant  is  effected  by 
the  circumnutation  of  the  arched  portion — that  is,  by  growth 
alternating  between  the  upper  and  lower  surface,  but  prepon- 
derant on  the  lower  surface,  with  some  little  lateral  movement. 

We  were  enabled  to  trace  the  movement  of  another  straight- 
ening shoot  for  a  longer  period  (owing  to  its  slower  growth  and 
to  its  having  been  placed  further  from  the  vertical  glass),  namely, 
from  the  early  morning  on  July  13th  to  late  in  the  evening  of  the 
16th.  During  the  whole  daytime  of  the  14th,  the  hook  straight- 
ened itself  very  little,  but  zigzagged  and  plainly  circumnutated 
about  nearly  the  same  spot.  By  the  16th  it  had  become  nearly 
straight,  and  the  tracing  was  no  longer  accurate,  yet  it  was 
manifest  that  there  was  still  a  considerable  amount  of  movement 
both  up  and  down  and  laterally;  for  the  crown  whilst  con- 
tinuing to  straighten  itself  occasionally  became  for  a  short  time 
more  curved,  causing  the  filament  to  descend  twice  during  the 
day. 

timitlria  Pfundii. — The  stiff  terminal  shoots  of  this  Legu- 
minous water-plant  from  Africa  project  so  as  to  make  a  rectangle 
with  the  stem  below ;  but  this  occurs  only  when  the  plants  are 
growing  vigorously,  for  when  kept  in  a  cool  place,  the  summits 
of  the  steins  become  straight,  as  they  likewise  did  at  the  close 
of  the  growing  season.  The  direction  of  the  rectangularly  bent 
part  is  independent  of  the  chief  source  of  light.  But  from 
observing  the  effects  of  placing  plants  in  the  dark,  in  which 
case  several  shoots  became  in  two  or  three  days  upright  or  nearly 
upright,  and  when  brought  back  into  the  light  again  became 
rectangularly  curved,  we  believe  that  the  bending  is  in  part 
due  to  apheliotropism,  apparently  somewhat  opposed  by  apogeo- 
tropism.  On  the  other  hand,  from  observing  the  effects  of  tying 
a  shoot  downwards,  so  that  the  rectangle  faced  upwards,  we  are 
led  to  believe  that  the  curvature  is  partly  due  to  epinasty.  As 
the  rectangularly  bent  portion  of  an  upright  stem  grows  older, 
the  lower  part  straightens  itself;  and  this  is  effected  through 
hyponasty.  He  who  has  read  Sachs'  recent  Essay  on  the  vertical 


276  MODIFIED   CIKCUMNUTATION.  CHAP.  V 

and  inclined  positions  of  the  parts  of  plants*  will  see  how  diffi- 
cult a  subject  this  is,  and  will  feel  no  surprise  at  our  expressing 
ourselves  doubtfully  in  this  and  other  such  cases. 

A  plant,  20  inches  in  height,  was  secured  to  a  stick  close 
beneath  the  curved  summit,  which  formed  rather  less  than  a 
rectangle  with  the  stem  below.  The  shoot  pointed  away  from  the 
observer ;  and  a  glass  filament  pointing  towards  the  vertical  glass 
on  which  the  tracing  was  made,  was  fixed  to  the  convex  surface  of 
the  curved  portion.  Therefore  the  descending  lines  in  the  figure 
represent  the  straightening  of  the  curved  portion  as  it  grew 
older.  The  tracing  (Fig.  123,  p.  274)  was  begun  at  9  A.M.  on 
July  10th;  the  filament  at  first  moved  but  little  in  a  zigzag  line, 
but  at  2  P.M.  it  began  rising  and  continued  to  do  so  till  9  P.M.  ; 
and  this  proves  that  the  terminal  portion  was  being  more  bent 
downwards.  After  9  P.M.  on  the  10th  an  opposite  movement 
commenced,  and  the  curved  portion  began  to  straighten  itself, 
and  this  continued  till  11.10  A.M.  on  the  12th,  but  was  interrupted 
by  some  small  oscillations  and  zigzags,  showing  movement  in 
different  directions.  After  11.10  A.M.  on  the  12th  this  part  of 
the  stem,  still  considerably  curved,  circumnutated  in  a  con- 
spicuous manner  until  nearly  3  P.M.  on  the  13th;  but  during  all 
this  time  a  downward  movement  of  the  filament  prevailed, 
caused  by  the  continued  straightening  of  the  stem.  By  the 
afternoon  of  the  13th,  the  summit,  which  had  originally  been 
deflected  more  than  a  right  angle  from  the  perpendicular,  had 
grown  so  nearly  straight  that  the  tracing  could  no  longer  be 
continued  on  the  vertical  glass.  There  can  therefore  be  no 
doubt  that  the  straightening  of  the  abruptly  curved  portion  of 
the  growing  stem  of  this  plant,  which  appears  to  be  wholly  due 
to  hyponasty,  is  the  result  of  modified  circumnutation.  We 
will  only  add  that  a  filament  was  fixed  in  a  different  manner 
across  the  curved  summit  of  another  plant,  and  the  same  general 
kind  of  movement  was  observed. 

Trifolium  repens. — In  many,  but  not  in  all  the  species  of  Tri- 
folium,  as  the  separate  little  flowers  wither,  the  sub-peduncles 
bend  downwards,  so  as  to  depend  parallel  to  the  upper  part  of 
the  main  peduncle.  In  Tr.  subterraneum  the  main  peduncle 
curves  downwards  for  the  sake  of  burying  its  capsules,  and  in 
this  species  the  sub-peduncles  of  the  separate  flowers  bend 


*  '  Ueber  Orthotrope  und  Pla-      ten  des  Bot.  Inst.,  in  Wurzburg,' 
giotrope  Pflanzentheile ;'  «Arbei-      Heft  ii.  1879,  p.  226. 


CHAP.  V.  -  EPINASTY   AND  HYPONASTY. 

Fig.  124. 


277 


L    \\ 


Trifolium  repens :  circumnu- 
tating  and  epinastic  move- 
ments of  the  sub-peduncle 
of  a  single  flower,  traced 
on  a  vertical  glass  under 
a  skylight,  in  A  from  11.30 
A.M.  Aug.  27th  to  7  A.M. 
30th;  in  B  from  7  A.M. 
Aug.  30th  to  a  little  after 
6  P.M.  Sept.  8th. 


278  MODIFIED  CIRCUMNUTATION.  CHAP.  V. 

upwards,  so  as  to  occupy  the  same  position  relatively  to  the 
upper  part  of  the  main  peduncle  as  in  Tr.  repens.  This  fact 
alone  would  render  it  probable  that  the  movements  of  the  sub- 
peduncles  in  Tr.  repens  were  independent  of  geotropism.  Never- 
theless, to  make  sure,  some  flower-heads  were  tied  to  little  sticks 
upside  down  and  others  in  a  horizontal  position;  their  sub- 
peduncles,  however,  all  quickly  curved  upwards  through  the 
action  of  heliotropism.  We  therefore  protected  some  flower- 
heads,  similarly  secured  to  sticks,  from  the  light,  and  although 
some  of  them  rotted,  many  of  their  sub-peduncles  turned  very 
slowly  from  their  reversed  or  from  their  horizontal  positions, 
so  as  to  stand  in  the  normal  manner  parallel  to  the  upper  part 
of  the  main  peduncle.  These  facts  show  that  the  movement  is 
independent  of  geotropism  or  apheliotropism ;  it  must  there- 
be  attributed  to  epinasty,  which  however  is  checked,  at  least  as 
long  as  the  flowers  are  young,  by  heliotropism.  Most  of  the 
above  flowers  were  never  fertilised  owing  to  the  exclusion  of 
bees ;  they  consequently  withered  very  slowly,  and  the  movements 
of  the  sub- peduncles  were  in  like  manner  much  retarded. 

To  ascertain  the  nature  of  the  movement  of  the  sub-peduncle, 
whilst  bending  downwards,  a  filament  was  fixed  across  the 
summit  of  the  calyx  of  a  not  fully  expanded  and  almost  upright 
flower,  nearly  in  the  centre  of  the  head.  The  main  peduncle 
was  secured  to  a  stick  close  beneath  the  head.  In  order  to  see 
the  marks  on  the  glass  filament,  a  few  flowers  had  to  be  cut 
away  on  the  lower  side  of  the  head.  The  flower-  under  obser- 
vation at  first  diverged  a  little  from  its  upright  position,  so  as 
to  occupy  the  open  space  caused  by  the  removal  of  the  adjoining 
flowers.  This  required  two  days,  after  which  time  a  new  tracing 
was  begun  (Fig.  124).  In  A  we  see  the  complex  circumnutating 
course  pursued  from  11.30  A.M.  Aug.  26th  to  7  A.M.  on  the 
30th.  The  pot  was  then  moved  a  very  little  to  the  right,  and 
the  tracing  (B)  was  continued  without  interruption  from  7  A.M. 
Aug.  30th  to  after  6  P.M.  Sept.  8th.  It  should  be  observed  that 
on  most  of  these  days,  only  a  single  dot  was  made  each  morning 
at  the  same  hour.  Whenever  the  flower  was  observed  carefully, 
as  on  Aug.  30th  and  Sept.  5th  and  6th,  it  was  found  to  be  cir- 
cumnutating over  a  small  space.  At  last,  on  Sept.  7th,  it 
began  to  bend  downwards,  and  continued  to  do  so  until  after 
6  P.M.  on  the  8th,  and  indeed  until  the  morning  of  the  9th,  when 
its  movements  could  no  longer  be  traced  on  the  vertical  glass. 
It  was  carefully  observed  during  the  whole  of  the  8th,  and  by 


CHAP.  Y.  EPINASTY  AND  HYPONASTY.  279 

10.30  P.M.  it  had  descended  to  a  point  lower  down  by  two-thirds 
of  the  length  of  the  figure  as  here  given ;  but  from  want  of  space 
the  tracing  has  been  copied  in  B,  only  to  a  little  after  6  P.M.  On 
the  morning  of  the  9th  the  flower  was  withered,  and  the  sub- 
peduncle  now  stood  at  an  angle  of  57°  beneath  the  horizon.  If 
the  flower  had  been  fertilised  it  would  have  withered  much 
sooner,  and  have  moved  much  more  quickly.  We  thus  see  that 
the  sub-peduncle  oscillated  up  and  down,  or  circumnutated, 
during  its  whole  downward  epinastic  course. 

The  sub-peduncles  of  the  fertilised  and  withered  flowers 
of  Oxalis  carnosa  likewise  bend  downwards  through  epinasty, 
as  will  be  shown  in  a  future  chapter;  and  theii  downward 
course  is  strongly  zigzag,  indicating  circumnutation. 

The  number  of  instances  in  which  various  organs 
move  through  epinasty  or  hyponasty,  often  in  com- 
bination with  other  forces,  for  the  most  diversified 
purposes,  seems  to  be  inexhaustibly  great ;  and  from 
the  several  cases  which  have  been  here  given,  we  may 
safely  infer  that  such  movements  are  due  to  modified 
circumnutation. 

13 


280  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 


CHAPTER  VI. 

MODIFIED  CIRCUMNUTATION  :  SLEEP  OR  NYCTITROPIC  MOVEMENTS, 
THEIR  USE:  SLEEP  OF  COTYLEDONS. 

Preliminary  sketch  of  the  sleep  or  nyctitropic  movements  of  leaves- 
Presence  of  pulvini — The  lessening  of  radiation  the  final  cause  of 
nyctitropic  movements — Manner  of  trying  experiments  on  leaves  of 
Oxalis,  Arachis,  Cassia,  Melilotus,  Lotus  and  Marsilea,  and  on  the 
cotyledons  of  Mimosa— Concluding  remarks  on  radiation  from  leaves 
— Small  differences  in  the  conditions  make  a  great  difference  in  the 
result — Description  of  the  nyctitropic  position  and  movements  of 
the  cotyledons  of  various  plants — List  of  species— Concluding 
remarks — Independence  of  the  nyctitropic  movements  of  the  leaves 
and  cotyledons  of  the  same  species — Reasons  for  believing  that  the 
movements  have  been  acquired  for  a  special  purpose. 

THE  so-called  sleep  of  leaves  is  so  conspicuous  a 
phenomenon  that  it  was  observed  as  early  as  the 
time  of  Pliny  ;*  and  since  Linnaeus  published  his 
famous  Essay,  '  Somnus  Plantarum,'  it  has  been  the 
subject  of  several  memoirs.  Many  flowers  close  at 
night,  and  these  are  likewise  said  to  sleep ;  but  we 
are  not  here  concerned  with  their  movements,  for 
although  effected  by  the  same  mechanism  as  in  the 
case  of  young  leaves,  namely,  unequal  growth  on  the 
opposite  sides  (as  first  proved  by  Pfeffer),  yet  they  differ 
essentially  in  being  excited  chiefly  by  changes  of 
temperature  instead  of  light ;  and  in  being  effected,  as 
far  as  we  can  judge,  for  a  different  purpose.  Hardly 
any  one  supposes  that  there  is  any  real  analogy 


*  Pfeffer  has  given  a  clear  and       riodigchen  Bewegungen  der  Blat 
interesting  sketch  of  the  history       torgaue,'  1875,  p.  163. 
of  this  subject  in  his  *  Die  Pe- 


CHAP.  VI.  SLEEP  MOVEMENTS.  281 

between  the  sleep  of  animals  and  that  of  plants,* 
whether  of  leaves  or  flowers.  It  seems,  therefore, 
advisable  to  give  a  distinct  name  to  the  so-called 
sleep-movements  of  plants.  These  have  also  generally 
been  confounded,  under  the  term  "  periodic,"  with  the 
slight  daily  rise  and  fall  of  leaves,  as  described  in  the 
fourth  chapter ;  and  this  makes  it  all  the  more  desir- 
able to  give  some  distinct  name  to  sleep-movements. 
Nyctitropism  and  nyctitropic,  i.e.  night-tuwaing,  may 
be  applied  both  to  leaves  and  flowers,  and  will  be 
occasionally  used  by  us ;  but  it  would  be  best  to  con- 
fine the  term  to  leaves.  The  leaves  of  some  few  plants 
move  either  upwards  or  downwards  when  the  sun  shines 
intensely  on  them,  and  this  movement  has  sometimes 
been  called  diurnal  sleep ;  but  we  believe  it  to  be  of 
an  essentially  different  nature  from  the  nocturnal 
movement,  and  it  will  be  briefly  considered  in  a 
future  chapter. 

The  sleep  or  nyctitropisin  of  leaves  is  a  large 
subject,  and  we  think  that  the  most  convenient  plan 
will  be  first  to  give  a  brief  account  of  the  position 
which  leaves  assume  at  night,  and  of  the  advantages 
apparently  thus  gained.  Afterwards  the  more  re- 
markable cases  will  be  described  in  detail,  with 
respect  to  cotyledons  in  the  present  chapter,  and  to 
leaves  in  the  next  chapter.  Finally,  it  will  be  shown 
that  these  movements  result  from  circumnutation, 
much  modified  and  regulated  by  the  alternations  of 
day  and  night,  or  light  and  darkness ;  but  that  they 
are  also  to  a  certain  extent  inherited. 

Leaves,  when  they  go  to  sleep,  move  either  upwards 
or  downwards,  or  in  the  case  of  the  leaflets  of  com- 


*  Ch.  Koyer  must,  however,  be       Nat.'  (5th   series),  Bot.  vol. 
cxcepted ;  see   «  Annalea  des  Sc.        1868,  p.  378. 


282  MODIFIED   CIRCUMXUTATION.  CHAP.  VI. 

pound  leaves,  forwards,  that  is,  towards  the  apex  of  the 
leaf,  or  backwards,  that  is,  towards  its  base ;  or,  again, 
they  may  rotate  on  their  own  axes  without  moving 
either  upwards  or  downwards.  But  in  almost  every 
case  the  plane  of  the  blade  is  so  placed  as  to  stand 
nearly  or  quite  vertically  at  night.  Therefore  the  apex, 
or  the  base,  or  either  lateral  edge,  may  be  directed 
towards  the  zenith.  Moreover,  the  upper  surface  of 
each  leaf,  jind  more  especially  of  each  leaflet,  is  often 
brought  into  close  contact  with  that  of  the  opposite 
one ;  and  this  is  sometimes  effected  by  singularly 
complicated  movements.  This  fact  suggests  that  the 
upper  surface  requires  more  protection  than  the  lower 
one.  For  instance,  the  terminal  leaflet  in  Trifolium, 
after  turning  up  at  night  so  as  to  stand  vertically, 
often  continues  to  bend  over  until  the  upper  surface  is 
directed  downwards  whilst  the  lower  surface  is  fully 
exposed  to  the  sky;  and  an  arched  roof  is  thus 
formed  over  the  two  lateral  leaflets,  which  have  their 
upper  surfaces  pressed  closely  together.  Here  we  have 
the  unusual  case  of  one  of  the  leaflets  not  standing 
vertically,  or  almost  vertically,  at  night. 

Considering  that  leaves  in  assuming  their  nycti- 
tropic  positions  often  move  through  an  angle  of 
90°;  that  the  movement  is  rapid  in  the  evening; 
that  in  some  cases,  as  we  shall  see  in  the  next 
chapter,  it  is  extraordinarily  complicated;  that  with 
certain  seedlings,  old  enough  to  bear  true  leaves, 
the  cotyledons  move  vertically  upwards  at  night, 
whilst  at  the  same  time  the  leaflets  move  ver- 
tically downwards;  and  that  in  the  same  genus 
the  leaves  or  cotyledons  of  some  species  move 
upwards,  whilst  those  of  other  species  move  down- 
wards ; — from  these  and  other  such  facts,  it  is  hardly 
possible  to  doubt  that  plants  must  derive  some 


CHAP.  VI  SLEEP  MOVEMENTS.  283 

great    advantage   from    such    remarkable    powers    of 
movement. 

The  nyctitropic  movements  of  leaves  and  cotyledons 
are  effected  in  two  ways,*  firstly,  by  means  of  pulvini 
which  become,  as  Pfeffer  has  shown,  alternately  more 
turgescent  on  opposite  sides ;  and  secondly,  by  in- 
creased growth  along  one  side  of  the  petiole  or 
midrib,  and  then  on  the  opposite  side,  as  was  first 
proved  by  Batalin.j  But  as  it  has  been  shown  by 
De  Yries  J  that  in  these  latter  cases  increased  growth 
is  preceded  by  the  increased  turgescence  of  the  cells, 
the  difference  between  the  above  two  means  of  move- 
ment is  much  diminished,  and  consists  chiefly  in  the 
turgescence  of  the  cells  of  a  fully  developed  pulvinus, 
not  being  followed  by  growth.  When  the  move- 
ments of  leaves  or  cotyledons,  furnished  with  a  pul- 
vinus and  destitute  of  one,  are  compared,  they  are  seen 
to  be  closely  similar,  and  are  apparently  effected  for 
the  same  purpose.  Therefore,  with  our  object  in  view, 
it  does  not  appear  advisable  to  separate  the  above  two 
sets  of  cases  into  two  distinct  classes.  There  is,  how- 
ever, one  important  distinction  between  them,  namely, 
that  movements  effected  by  growth  on  the  alternate 
sides,  are  confined  to  young  growing  leaves,  whilst  those 
effected  by  means  of  a  pulvinus  last  for  a  long  time. 
We  have  already  seen  well-marked  instances  of  this 
latter  fact  with  cotyledons,  and  so  it  is  with  leaves,  as 
has  been  observed  by  Pfeffer  and  by  ourselves.  The 
long  endurance  of  the  nyctitropic  movements  when 
effected  by  the  aid  of  pulvini  indicates,  in  addition  to 
the  evidence  already  advanced,  the  functional  iinr>ort- 


*  This    distinction    was    first  Dassen  in  1837. 
pointed  out  (according  to  Pfefter,  t  *  Flora/  1873,  p.  433. 

4  Die   Periodischeu   Bewegungen  j  '  Bot.     Zeitung,'     1879,   Doc. 

der  Blattorgaue,'  1875,  p.  1G1)  by  19 ih,  p.  830. 


284  MODIFIED  CIECUMNUTATION.  CHAP.  VI 

ance  of  such  movements  to  the  plant.  There  is  another 
difference  between  the  two  sets  of  cases,  namely,  that 
there  is  never,  or  very  rarely,  any  torsion  of  the 
leaves,  excepting  when  a  pulvinus  is  present ;  *  but 
this  statement  applies  only  to  periodic  and  nyctitropic 
movements,  as  may  be  inferred  from  other  cases  given 
by  Frank.f 

The  fact  that  the  leaves  of  many  plants  place 
themselves  at  night  in  widely  different  positions  from 
what  they  hold  during  the  day,  but  with  the  one 
point  in  common,  that  their  upper  surfaces  avoid 
facing  the  zenith,  often  with  the  additional  fact  that 
they  come  into  close  contact  with  opposite  leaves  or 
leaflets,  clearly  indicates,  as  it  seems  to  us,  that  the 
object  gained  is  the  protection  of  the  upper  sur- 
faces from  being  chilled  at  night  by  radiation.  There 
is  nothing  improbable  in  the  upper  surface  needing 
protection  more  than  the  lower,  as  the  two  differ  in 
function  and  structure.  All  gardeners  know  that 
plants  suffer  from  radiation.  It  is  this  and  not 
cold  winds  which  the  peasants  of  Southern  Europe 
fear  for  their  olives.J  Seedlings  are  often  protected 
from  radiation  by  a  very  thin  covering  of  straw ;  and 
fruit-trees  on  walls  by  a  few  fir-branches,  or  even  by  a 
fishing-net,  suspended  over  them.  There  is  a  variety 
of  the  gooseberry,§  the  flowers  of  which  from  being 
produced  before  the  leaves,  are  not  protected  by 
them  from  radiation,  and  consequently  often  fail  to 
yield  fruit.  An  excellent  observer  ||  has  remarked 

*  Pfeffer,  •  Die  Period.  Beweg.  Dew,'  remarks  that  an  exposed 

der  Blattorgane,'  3875,  p.  159.  thermometer  rises  as  soon  as  even 

f  'Die  Nat.  Wagerechte  Rich-  a  fleecy  cloud,  high  in  the  sky, 

tung  von  Pflanzentheilen1,'  1870,  pastes  over  the  zenith. 

p.  52.  §  '  Loud  oil's  Gardener's  Mag.,' 

J  Martins  in   'Bull.  Soc.  Bot.  vol.  iv.  1828,  p.  112. 

de     France,'     torn.     xix.    1872.  ||  Mr.    Rivers    in   'Gardener's 

Wells,  in   his  famous  '  Essay  on  Chron.,'  1866,  p.  732. 


CHAP.  VI.  USE   OF   SLEEP  MOVEMENTS.  285 

that  one  variety  of  the  cherry  has  the  petals  of  its 
flowers  much  curled  backwards,  and  after  a  severe 
frost  all  the  stigmas  were  killed  ;  whilst  at  the  same 
time,  in  another  variety  with  incurved  petals,  the 
stigmas  were  not  in  the  least  injured. 

This  view  that  the  sleep  of  leaves  saves  them  from 
being  chilled  at  night  by  radiation,  would  no  doubt 
have  occurred  to  Linnaeus,  had  the  principle  of  radia- 
tion been  then  discovered ;  for  he  suggests  in  many 
parts  of  his  *  Somnus  Plantarum '  that  the  position  of 
the  leaves  at  night  protects  the  young  stems  and 
buds,  and  often  the  young  inflorescence,  against  cold 
winds.  We  are  far  from  doubting  that  an  additional 
advantage  may  be  thus  gained ;  and  we  have  observed 
with  several  plants,  for  instance,  Desmodium  gyrans, 
that  whilst  the  blade  of  the  leaf  sinks  vertically  down  at 
night,  the  petiole  rises,  so  that  the  blade  has  to  move 
through  a  greater  angle  in  order  to  assume  its  vertical 
position  than  would  otherwise  have  been  necessary ;  but 
with  the  result  that  all  the  leaves  on  the  same  plant 
are  crowded  together  as  if  for  mutual  protection. 

We  doubted  at  first  whether  radiation  would  affect 
in  any  important  manner  objects  so  thin  as  are  many 
cotyledons  and  leaves,  and  more  especially  affect  dif- 
ferently their  upper  and  lower  surfaces ;  for  although 
the  temperature  of  their  upper  surfaces  would  un- 
doubtedly fall  when  freely  exposed  to  a  clear  sky,  yet 
we  thought  that  they  would  so  quickly  acquire  by 
conduction  the  temperature  of  the  surrounding  air, 
that  it  could  hardly  make  any  sensible  difference 
to  them,  whether  they  stood  horizontally  and  radiated 
into  the  open  sky,  or  vertically  and  radiated  chiefly 
in  a  lateral  direction  towards  neighbouring  plants  and 
other  objects.  We  endeavoured,  therefore,  to  ascer- 
tain something  on  this  head  by  preventing  the  leaves 


286  MODIFIED  CIRCUMNUTATION.  CHAP.  VI 

of  several  plants  from  going  to  sleep,  and  by  exposing 
to  a  clear  sky  when  the  temperature  was  beneath 
the  freezing-point,  these,  as  well  as  the  other  leaves 
on  the  same  plants  which  had  already  assumed  their 
nocturnal  vertical  position.  Our  experiments  show 
that  leaves  thus  compelled  to  remain  horizontal  at 
night,  suffered  much  more  injury  from  frost  than 
those  which  were  allowed  to  assume  their  normal 
vertical  position.  It  may,  however,  be  said  that 
conclusions  drawn  from  such  observations  are  not 
applicable  to  sleeping  plants,  the  inhabitants  of 
countries  where  frosts  do  not  occur.  But  in  every 
country,  and  at  all  seasons,  leaves  must  be  exposed  to 
nocturnal  chills  through  radiation,  which  might  be  in 
some  degree  injurious  to  them,  and  which  they  would 
escape  by  assuming  a  vertical  position. 

In  our  experiments,  leaves  were  prevented  from 
assuming  their  nyctitropic  position,  generally  by 
being  fastened  with  the  finest  entomological  pins 
(which  did  not  sensibly  injure  them)  to  thin  sheets 
of  cork  supported  on  sticks.  But  in  some  instances 
they  were  fastened  down  by  narrow  strips  of  card, 
and  in  others  by  their  petioles  being  passed  through 
slits  in  the  cork.  The  leaves  were  at  first  fastened 
close  to  the  cork,  for  as  this  is  a  bad  conductor,  and  as 
the  leaves  were  not  exposed  for  long  periods,  we  thought 
that  the  cork,  which  had  been  kept  in  the  house,  would 
very  slightly  warm  them ;  so  that  if  they  were  injured 
by  the  frost  in  a  greater  degree  than  the  free  vertical 
leaves,  the  evidence  would  be  so  much  the  stronger 
that  the  horizontal  position  was  injurious.  But  we 
found  that  when  there  was  any  slight  difference  in  the 
result,  which  could  be  detected  only  occasionally,  the 
leaves  which  had  been  fastened  closely  down  suffered 
rather  more  than  those  fastened  with  very  long  and 


CHAP.  VI.  USE   OF  SLEEP  MOVEMENTS.  287 

thin  pins,  so  as  to  stand  from  i  to  f  inch  above  the 
cork.  This  difference  in  the  result,  which  is*dn  itself 
curious  as  showing  what  a  very  slight  difference  in 
the  conditions  influences  the  amount  of  injury  in- 
flicted, may  be  attributed,  as  we  believe,  to  the  sur- 
rounding warmer  air  not  circulating  freely  beneath  the 
closely  pinned  leaves  and  thus  slightly  warming  them. 
This  conclusion  is  supported  by  some  analogous  facts 
hereafter  to  be  given. 

We  will  now  describe  in  detail  the  experiments 
which  were  tried.  These  were  troublesome  from  our 
not  being  able  to  predict  how  much  cold  the  leaves  of 
the  several  species  could  endure.  Many  plants  had 
every  leaf  killed,  both  those  which  were  secured  in 
a  horizontal  position  and  those  which  were  allowed  to 
sleep — that  is,  to  rise  up  or  sink  down  vertically. 
Others  again  had  not  a  single  leaf  in  the  least  in- 
jured, and  these  had  to  be  re-exposed  either  for  a 
longer  time  or  to  a  lower  temperature. 

Oxalis  acetosella. — A  very  large  pot,  thickly  covered  with 
between  300  and  400  leaves,  had  been  kept  all  winter  in  the 
greenhouse.  Seven  leaves  were  pinned  horizontally  open, 
and  were  exposed  on  March  16th  for  2  h.  to  a  clear  sky,  the 
temperature  on  the  surrounding  grass  being  -  4°  C.  (24°  to 
25°  F.).  Next  morning  all  seven  leaves  were  found  quite 
killed,  so  were  many  of  the  free  ones  which  had  previously 
gene  to  sleep,  and  about  100  of  them,  either  dead  or  browned 
and  injured,  were  picked  off.  Some  leaves  showed  that  they 
had  been  slightly  injured  by  not  expanding  during  the  whole 
of  the  next  day,  though  they  afterwards  recovered.  As  all  the 
leaves  which  were  pinned  open  were  killed,  and  only  about  a 
third  or  fourth  of  the  others  were  either  killed  or  injured,  we 
had  some  little  evidence  that  those  which  were  prevented  from 
assuming  their  vertically  dependent  position  suffered  most. 

The  following  night  (17th)  was  clear  and  almost  equally  cold 
(—  3°  to  —  4°  C.  on  the  grass),  and  the  pot  was  again  exposed, 
but  this  time  for  only  30  m.  Eight  leaves  had  been  pinned  out, 


MODIFIED   CIKCUMNUTATION.  CHAP.  VI. 

and  in  the  morning  two  of  them  were  dead,  whilst  not  a  single 
other  leaf  on  the  many  plants  was  even  injured. 

On  the  23rd  the  pot  was  exposed  for  1  h.  30  m.,  the  tempera- 
ture on  the  grass  being  only  -  2°  C.,  and  not  one  leaf  was 
injured:  the  pinned  open  leaves,  However,  all  stood  from 
$  to  f  of  an  inch  above  the  cork. 

On  the  24th  the  pot  was  again  placed  on  the  ground  and 
exposed  to  a  clear  sky  for  between  35  m.  and  40  m.  By  a  mis- 
take the  thermometer  was  left  on  an  adjoining  sun-dial  3  feet 
high,  instead  of  being  placed  on  the  grass ;  it  recorded  25°  to 
26°  F.  (-  3-3°  to  -  3-8°  C.),  but  when  looked  at  after  1  h.  had 
fallen  to  22°  F.  (-  5'5° C.);  so  that  the  pot  was  perhaps  exposed 
to  rather  a  lower  temperature  than  on  the  two  first  occasions. 
Eight  leaves  had  been  pinned  out,  some  close  to  the  cork  and 
some  above  it,  and  on  the  following  morning  five  of  them  (i.e. 
63  per  cent.)  were  found  killed.  By  counting  a  portion  of  the 
leaves  we  estimated  that  about  250  had  been  allowed  to  go  to 
sleep,  and  of  these  about  20  were  killed  (i.e.  only  8  per  cent.), 
and  about  30  injured. 

Considering  these  cases,  there  can  be  no  doubt  that  the 
leaves  of  this  Oxalis,  when  allowed  to  assume  their  normal 
vertically  dependent  position  at  night,  suffer  much  less  from 
frost  than  those  (23  in  number)  which  had  their  upper  surfaces 
exposed  to  the  zenith. 

Oxalis  carnosa. — A  plant  of  this  Chilian  species  was  exposed 
for  30  m.  to  a  clear  sky,  the  thermometer  on  the  grass  standing 
at  —  2°  C ,  with  some  of  its  leaves  pinned  open,  and  not  one  leaf 
on  the  whole  bushy  plant  was  in  the  least  injured.  On  the 
16th  of  March  another  plant  was  similarly  exposed  for  30  m., 
when  the  temperature  on  the  grass  was  only  a  little  lower,  viz., 
—  3°  to  —  4°  C.  Six  of  the  leaves  had  been  pinned  open,  and 
next  morning  five  of  them  were  found  much  browned.  The 
plant  was  a  large  one,  and  none  of  the  free  leaves,  which 
were  asleep  and  depended  vertically,  were  browned,  excepting 
four  very  young  ones.  But  three  other  leaves,  though  not 
browned,  were  in  a  rather  flaccid  condition,  and  retained  their 
nocturnal  position  during  the  whole  of  the  following  day.  In 
this  case  it  was  obvious  that  the  leaves  which  were  exposed  hori- 
zontally to  the  zenith  suffered  most.  This  same  pot  was  after- 
wards exposed  for  35-40  m.  on  a  slightly  colder  night,  and 
every  leaf,  both  the  pinned  open  and  the  free  ones,  was  killed. 
It  may  be  added  that  two  pots  of  0.  corniculata  (var.  Atro- 


CHAP.  VI.  USE   OF   SLEEP   MOVEMENTS.  289 

purpurea)  were  exposed  for  2  h.  and  3  h.  to  a  clear  sky  with  the 
temp,  on  grass  —  2°  C.,  and  none  of  the  leaves,  whether  free  or 
pinned  open,  were  at  all  injured. 

Arachis  hypogcea. — Some  plants  in  a  pot  were  exposed  at  night 
for  30m.  to  a  clear  sky,  the  temperature  on  the  surrounding 
grass  being  —  2°  C.,  and  on  two  nights  afterwards  they  were  again 
exposed  to  the  same  temperature,  but  this  time  during  1  h.  30  m. 
On  neither  occasion  was  a  single  leaf,  whether  pinned  open  or 
free,  injured ;  and  this  surprised  us  much,  considering  its  native 
tropical  African  home.  Two  plants  were  next  exposed  (March 
16th)  for  30  m.  to  a  clear  sky,  the  temperature  of  the  surrounding 
grass  being  now  lower,  viz.,  between  —  3°  and  —  4°  C.,  and  all 
four  pinned-open  leaves  were  killed  and  blackened.  These  two 
plants  bore  22  other  and  free  leaves  (excluding  some  very  young 
bud-like  ones)  and  only  two  of  these  were  killed  and  three  some- 
what injured ;  that  is,  23  per  cent,  were  either  killed  or  injured, 
whereas  all  four  pinned  open  leaves  were  utterly  killed. 

On  another  night  two  pots  with  several  plants  were  exposed 
for  between  35  m.  and  40  m.  to  a  clear  sky,  and  perhaps  to  a 
rather  lower  temperature,  for  a  thermometer  on  a  dial,  3  feet 
high,  close  by  stood  at  —  3'3°  to  —  3*8°  C.  In  one  pot  three 
leaves  were  pinned  open,  and  all  were  badly  injured ;  of  the 
44  free  leaves,  26  were  injured,  that  is,  59  per  cent.  In  the 
other  pot  3  leaves  were  pinned  open  and  all  were  killed ;  four 
other  leaves  were  prevented  from  sleeping  by  narrow  strips  of 
stiff  paper  gummed  across  them,  and  all  were  killed ;  of  24  free 
leaves,  10  were  killed,  2  much  injured,  and  12  unhurt ;  that  is, 
50  per  cent,  of  the  free  leaves  were  either  killed  or  much  in- 
jured. Taking  the  two  pots  together,  we  may  say  that  rather 
more  than  half  of  the  free  leaves,  which  were  asleep,  were  either 
killed  or  injured,  whilst  all  the  ten  horizontally  extended  leaves, 
which  had  been  prevented  from  going  to  sleep,  were  either  killed 
or  much  injured. 

Cassia  floribunda. — A  bush  was  exposed  at  night  for  40  m.  to 
a  clear  sky,  the  temperature  on  the  surrounding  grass  being 
—  2°  C.,  and  not  a  leaf  was  injured.*  It  was  again  exposed  on 


*  Cassia  Ixvigata  was  exposed  injured.     But  when  C.  Isemgata 

to  a  clear  sky  for  35  m.,  and  C.  was  exposed  for  1  h.,  the  temp. 

cdllianiha  (a  Guiana  species)  for  on   the  surrounding  grass  being 

CO   m.,   the   temperature   on   the  between  —  3°  and  —  4°  C.,  every 

surrounding  grass  being  —  2°  C.,  leaf  was  killed, 
and    neither  were  in    the  least 


290  MODIFIED  CIRCUMNUTATION.  CHAP.  VI. 

another  night  for  1  h.,  when  the  temperature  of  the  grass  was 
—  4°  C. ;  and  now  all  the  leaves  on  a  large  bush,  whether  pinned 
flat  open  or  free,  were  killed,  blackened,  and  shrivelled,  with 
the  exception  of  those  on  one  small  branch,  low  down,  which 
was-  very  slightly  protected  by  the  leaves  on  the  branches 
above.  Another  tall  bush,  with  four  of  its  large  compound 
leaves  pinned  out  horizontally,  was  afterwards  exposed  (temp, 
of  surrounding  grass  exactly  the  same,  viz.,  —  4°  C.),  but  only 
for  30  m.  On  the  following  morning  every  single  leaflet  on 
these  four  leaves  was  dead,  with  both  their  upper  and  lower 
surfaces  completely  blackened.  Of  the  many  free  leaves  on  the 
bush,  only  seven  were  blackened,  and  of  these  only  a  single  one 
(which  was  a  younger  and  more  tender  leaf  than  any  of  the 
pinned  ones)  had  both  surfaces  of  the  leaflets  blackened.  The 
contrast  in  this  latter  respect  was  well  shown  by  a  free  leaf,  which 
stood  between  two  pinned-open  ones ;  for  these  latter  had  the 
lower  surfaces  of  their  leaflets  as  black  as  ink,  whilst  the  inter- 
mediate free  leaf,  though  badly  injured,  still  retained  a  plain 
tinge  of  green  on  the  lower  surface  of  the  leaflets.  This  bush 
exhibited  in  a  striking  manner  the  evil  effects  of  the  leaves  not 
being  allowed  to  assume  at  night  their  normal  dependent  posi- 
tion ;  for  had  they  all  been  prevented  from  doing  so,  assuredly 
every  single  leaf  on  the  bush  would  have  been  utterly  killed  by 
this  exposure  of  only  30  m.  The  leaves  whilst  sinking  down- 
wards in  the  evening  twist  round,  so  that  the  upper  surface  is 
turned  inwards,  and  is  thus  better  protected  than  the  outwardly 
turned  lower  surface.  Nevertheless,  it  was  always  the  upper 
surface  which  was  more  blackened  than  the  lower,  whenever 
any  difference  could  be  perceived  between  them ;  but  whether  this 
was  due  to  the  cells  near  the  upper  surface  being  more  tender, 
or  merely  to  their  containing  more  chlorophyll,  we  do  not  know. 
Mdilutus  ojficinalis. — A  large  pot  with  many  plants,  which 
had  been  kept  during  the  winter  in  the  greenhouse,  was  exposed 
during  5h.  at  night  to  a  slight  frost  and  clear  sky.  Four 
leaves  had  been  pinned  out,  and  these  died  after  a  few  days ; 
but  so  did  many  of  the  free  leaves.  Therefore  nothing  certain 
could  be  inferred  from  this  trial,  though  it  indicated  that  the 
Horizontally  extended  leaves  suffered  most.  Another  large  pot 
with  many  plants  was  next  exposed  for  1  h.,  the  temperature  on 
the  surrounding  grass  being  lower,  viz.,  —  3°  to  —  4°  C.  Ten 
leaves  had  been  pinned  out,  and  the  result  was  striking,  for 
on  the  following  morning  all  these  were  found  much  injured  or 


CHAP.  VI.  USE   OF   SLEEP  MOVEMENTS.  291 

killed,  and  none  of  the  many  free  leaves  on  the  several  plants 
were  at  all  injured,  with  the  doubtful  exception  of  two  or 
three  very  young  ones. 

Melilotus  Italica. — Six  leaves  were  pinned  out  horizontally, 
three  with  their  upper  and  three  with  their  lower  surfaces  turned 
to  the  zenith.  The  plants  were  exposed  for  5  h.  to  a  clear  sky, 
the  temperature  on  ground  being  about  —  1°  C.  Next  morning 
the  six  pinned-open  leaves  seemed  more  injured  even  than  the 
younger  and  more  tender  free  ones  on  the  same  branches.  The 
exposure,  however,  had  been  too  long,  for  after  an  interval  of 
some  days  many  of  the  free  leaves  seemed  in  almost  as  bad  a 
condition  as  the  pinned-out  ones.  It  was  not  possible  to  decide 
whether  the  leaves  with  their  upper  or  those  with  their  lower 
surfaces  turned  to  the  zenith  had  suffered  most. 

Melilotus  suaveolens. — Some  plants  with  8  leaves  pinned  out 
were  exposed  to  a  clear  sky  during  2  h.,  the  temperature  on  the 
surrounding  grass  being  —  2°  C.  Next  morning  6  out  of  these 
8  leaves  were  in  a  flaccid  condition.  There  were  about  150  free 
leaves  on  the  plant,  and  none  of  these  were  injured,  except  2  or  3 
very  young  ones.  But  after  two  days,  the  plants  having  been 
brought  back  into  the  greenhouse,  the  6  pinned-out  leaves  all 
recovered. 

Melilotus  Taurtca. — Several  plants  were  exposed  for  5  h,  during 
two  nights  to  a  clear  sky  and  slight  frost,  accompanied  by  some 
wind ;  and  5  leaves  which  had  been  pinned  out  suffered  more 
than  those  both  above  and  below  on  the  same  branches  which 
had  gone  to  sleep.  Another  pot,  which  had  likewise  been  kept 
in  the  greenhouse,  was  exposed  for  35-40  m.  to  a  clear  sky, 
the  temperature  of  the  surrounding  grass  being  between  —  3°  and 

—  4°  C.    Nine  leaves  had  been  pinned  out,  and  all  of  these  were 
killed.    On  the  same  plants  there  were  210  free  leaves,  which 
had  been  allowed  to  go  to  sleep,  and  of  these  about  80  were 
killed,  i.e.  only  38  per  cent. 

Melilotus  Petitpitrrcana. — The  plants  were  exposed  to  a  clear 
sky  for  35-40  m. :  temperature  on  surrounding  grass  —  3°  to 

—  4°  C.    Six  leaves  had  been  pinned  out  so  as  to  stand  about 
i  inch  above  the  cork,  and  four  had  been  pinned  close  to  it. 
These  10  leaves  were  all  killed,  but  the  closely  pinned  ones 
suffered  most,  as  4  of  the  6  which  stood  above  the  cork  still 
retained  small  patches   of  a    green  colour.      A  considerable 
number,  but  not  nearly  all,  of  the  free  leaves,  were  killed  or 
much  injured,  whereas  all  the  pinned  out  ones  were  killed. 


292  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

Melilotus  macrorrhiza. — The  plants  were  exposed  in  the  same 
manner  as  in  the  last  case.  Six  leaves  had  been  pinned  out 
horizontally,  and  five  of  them  were  killed,  that  is,  83  per  cent. 
We  estimated  that  there  were  200  free  leaves  on  the  plants,  and 
of  these  about  50  were  killed  and  20  badly  injured,  so  that  about 
35  per  cent,  of  the  free  leaves  were  killed  or  injured. 

Lotus  aristata. — Six  plants  were  exposed  for  nearly  5  h.  to  a 
clear  sky ;  temperature  on  surrounding  grass  —  1'5°  C.  Four 
leaves  had  been  pinned  out  horizontally,  and  2  of  these  suffered 
more  than  those  above  or  below  on  the  same  branches,  which 
had  been  allowed  to  go  to  sleep.  It  is  rather  a  remarkable  fact 
that  some  plants  of  Lotus  Jacobceus,  an  inhabitant  of  so  hot  a 
country  as  the  Cape  Verde  Islands,  were  exposed  one  night  to  a 
clear  sky,  with  the  temperature  of  the  surrounding  grass  —  2°  C., 
and  on  a  second  night  for  30  m.  with  the  temperature  of 
the  grass  between  —  3°  and  —  4°  C.,  and  not  a  single  leaf,  either 
the  pinned-out  or  free  ones,  was  in  the  least  injured. 

Marsilea  quadrifoliata. — A  large  plant  of  this  species — the 
only  Cryptogamic  plant  known  to  sleep— with  some  leaves  pinned 
open,  was  exposed  for  1  h.  35  m.  to  a  clear  sky,  the  temperature 
on  the  surrounding  ground  being  —  2°  C.,  and  not  a  single  leaf 
was  injured.  After  an  interval  of  some  days  the  plant  was  again 
exposed  for  1  h.  to  a  clear  sky,  with  the  temperature  on  the 
surrounding  ground  lower,  viz.,  —  4°  C.  Six  leaves  had  been 
pinned  out  horizontally,  and  all  of  them  were  utterly  killed. 
The  plant  had  emitted  long  trailing  stems,  and  these  had  been 
wrapped  round  with  a  blanket,  so  as  to  protect  them  from  the 
frozen  ground  and  from  radiation;  but  a  very  large  number 
of  leaves  were  left  freely  exposed,  which  had  gone  to  sleep, 
and  of  these  only  12  were  killed.  After  another  interval,  the 
plant,  with  9  leaves  pinned  out,  was  again  exposed  for  1  h.,  the 
temperature  on  the  ground  being  again  —  4°  C.  Six  of  the  leaves 
were  killed,  and  one  which  did  not  at  first  appear  injured  after- 
wards became  streaked  with  brown.  The  trailing  branches,  which 
rested  on  the  frozen  ground,  had  one-half  or  three-quarters  of  their 
leaves  killed,  but  of  the  many  other  leaves  on  the  plant,  which 
alone  could  be  fairly  compared  with  the  pinned-out  ones,  none 
appeared  at  first  sight  to  have  been  killed,  but  on  careful  search 
12  were  found  in  this  state.  After  another  interval,  the  plant 
with  9  leaves  pinned  out,  was  exposed  for  35-40  m.  to  a  clear 
sky  and  to  nearly  the  same,  or  perhaps  a  rather  lower,  tempera- 
ture (for  the  thermometer  by  an  accident  had  been  left  on  a 


CHAP.  VI.  USE   OF   SLEEP  MOVEMENTS.  293 

sun-dial  close  by),  and  8  of  these  leaves  were  killed.  Of  the  free 
leaves  (those  on  the  trailing  branches  not  being  considered),  a 
good  many  were  killed,  but  their  number,  compared  with  the 
uninjured  ones,  was  small.  Finally,  taking  the  three  trials 
together,  24  leaves,  extended  horizontally,  were  exposed  to  the 
zenith  and  to  unobstructed  radiation,  and  of  these  20  were 
killed  and  1  injured ;  whilst  a  relatively  very  small  proportion 
of  the  leaves,  which  had  been  allowed  to  go  to  sleep  with  their 
leaflets  vertically  dependent,  were  killed  or  injured. 

The  cotyledons  of  several  plants  were  prepared  for  trial,  but 
the  weather  was  mild  and  we  succeeded  only  in  a  single  instance 
in  having  seedlings  of  the  proper  age  on  nights  which  were 
clear  and  cold.  The  cotyledons  of  6  seedlings  of  Mimosa  pudica 
were  fastened  open  on  cork,  and  were  thus  exposed  for  1  h.  45  m. 
to  a  clear  sky,  with  the  temperature  on  the  surrounding  ground 
at  29°  F. ;  of  these,  3  were  killed.  Two  other  seedlings,  after 
their  cotyledons  had  risen  up  and  had  closed  together,  were 
bent  over  and  fastened  so  that  they  stood  horizontally,  with  the 
lower  surface  of  one  cotyledon  fully  exposed  to  the  zenith,  and 
both  were  killed.  Therefore  of  the  8  seedlings  thus  tried  5,  or 
more  than  half,  were  killed.  Seven  other  seedlings,  with  their 
cotyledons  in  their  normal  nocturnal  position,  viz.,  vertical  and 
closed,  were  exposed  at  the  same  time,  and  of  these  only  2  were 
killed.*  Hence  it  appears,  as  far  as  these  few  trials  tell  anything, 
that  the  vertical  position  at  night  of  the  cotyledons  of  Mimosa 
pudica  protects  them  to  a  certain  degree  from  the  evil  effects  of 
radiation  and  cold. 

Concluding  Remarks  on  the  Radiation  from  Leaves 
at  Night. — We  exposed  on  two  occasions  during  the 
summer  to  a  clear  sky  several  pinned-open  leaflets 
of  Trifolium  pratense,  which  naturally  rise  at  night, 
and  of  Oxalis  purpurea,  which  naturally  sink  at  night 
(the  plants  growing  out  of  doors),  and  looked  at 


*  We     were     surprised     that  It  may  be  added  that  seedlings  of 

young  seedlings  of  so  tropical  a  the  Indian  Cassia  pubescens  were 

plant  as  Mimosa  pudica  were  able  exposed  for  1  h.  30  m.  to  a  clear 

to  resifet,  as  well  as  they  did,  ex-  sky,  with  the  temp,  on  the  sur- 

posure  for  1  hr.  45  m.  to  a  clear  rounding  ground  at  —  2°  C.,  and 

Kky,  the  temperature  on  the  sur-  they  were  not  in  the  least  injured, 
rounding  ground  being    29°  F. 


294  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

them  early  on  several  successive  mornings,  after  they 
had  assumed  their  diurnal  positions.  The  difference 
in  the  amount  of  dew  on  the  pinned-open  leaflets 
and  on  those  which  had  gone  to  sleep  was  generally 
conspicuous ;  the  latter  being  sometimes  absolutely 
dry,  whilst  the  leaflets  which  had  been  horizontal 
were  coated  with  large  beads  of  dew.  This  shows  how 
much  cooler  the  leaflets  fully  exposed  to  the  zenith 
must  have  become,  than  those  which  stood  almost 
vertically,  either  upwards  or  downwards,  during  the 
night. 

From  the  several  cases  above  given,  there  can  be  no 
doubt  that  the  position  of  the  leaves  at  night  affects 
their  temperature  through  radiation  to  such  a  degree, 
that  when  exposed  to  a  clear  sky  during  a  frost,  it  is  a 
question  of  life  and  death.  We  may  therefore  admit 
as  highly  probable,  seeing  that  their  nocturnal  posi- 
tion is  so  well  adapted  to  lessen  radiation,  that  the 
object  gained  by  their  often  complicated  sleep  move- 
ments, is  to  lessen  the  degree  to  which  they  are 
chilled  at  night.  It  should  be  kept  in  mind  that 
it, is  especially  the  upper  surface  which  is  thus  pro- 
tected, as  it  is  never  directed  towards  the  zenith,  and 
is  often  brought  into  close  contact  with  the  upper 
surface  of  an  opposite  leaf  or  leaflet. 

We  failed  to  obtain  sufficient  evidence,  whether 
the  better  protection  of  the  upper  surface  has  been 
gained  from  its  being  more  easily  injured  than  the 
lower  surface,  or  from  its  injury  being  a  greater  evil 
to  the  plant.  That  there  is  some  difference  in  consti- 
tution between  the  two  surfaces  is  shown  by  the  follow- 
ing cases.  Cassia  floribunda  was  exposed  to  a  clear  sky 
on  a  sharp  frosty  night,  and  several  leaflets  which 
had  assumed  their  nocturnal  dependent  position  with 
their  lower  surfaces  turned  outwards  so  as  to  be 


CHAP.  VI.  USE   OF   SLEEP  MOVEMENTS.  295 

exposed  obliquely  to  the  zenith,  nevertheless  had  these 
lower  surfaces  less  blackened  than  the  upper  surfaces 
which  were  turned  inwards  and  were  in  close  contact 
with  those  of  the  opposite  leaflets.  Again,  a  pot 
full  of  plants  of  Trifolium  resupinatum,  which  had 
been  kept  in  a  warm  room  for  three  days,  was  turned 
out  of  doors  (Sept.  21st)  on  a  clear  and  almost  frosty 
night.  Next  morning  ten  of  the  terminal  leaflets  were 
examined  as  opaque  objects  under  the  microscope. 
These  leaflets,  in  going  to  sleep,  either  turn  vertically 
upwards,  or  more  commonly  bend  a  little  over  the 
lateral  leaflets,  so  that  their  lower  surfaces  are  more 
exposed  to  the  zenith  than  their  upper  surfaces. 
Nevertheless,  six  of  these  ten  leaflets  were  distinctly 
yellower  on  the  upper  than  on  the  lower  and  more 
exposed  surface.  In  the  remaining  four,  the  result 
was  not  so  plain,  but  certainly  whatever  difference 
there  was  leaned  to  the  side  of  the  upper  surface 
having  suffered  most. 

It  has  been  stated  that  some  of  the  leaflets  experi- 
mented on  were  fastened  close  to  the  cork,  and  others 
at  a  height  of  from  J  to  £  of  an  inch  above  it ;  and 
that  whenever,  after  exposure  to  a  frost,  any  difference 
could  be  detected  in  their  states,  the  closely  pinned 
ones  had  suffered  most.  We  attributed  this  difference 
to  the  air,  not  cooled  by  radiation,  having  been  pre- 
vented from  circulating  freely  beneath  the  closely 
pinned  leaflets.  That  there  was  really  a  difference  in 
the  temperature  of  leaves  treated  in  these  two  dif- 
ferent methods,  was  plainly  shown  on  one  occasion ; 
for  after  the  exposure  of  a  pot  with  plants  of  Melilotus 
dentaia  for  2  h.  to  a  clear  sky  (the  temperature  on  the 
surrounding  grass  being  —  2°  C.),  it  was  manifest  that 
more  dew  had  congealed  into  hoar-frost  on  the  closely 
pinned  leaflets,  than  on  those  which  stood  horizontally 


296  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

a  little  above  the  cork.  Again,  the  tips  of  some  few 
leaflets,  which  had  been  pinned  close  to  the  cork,  pro- 
ected  a  little  beyond  the  edge,  so  that  the  air  could 
circulate  freely  round  them.  This  occurred  with  six 
leaflets  of  Oxalis  acetosella,  and  their  tips  certainly 
suffered  rather  less  than  the  rest  of  the  same  leaflets  ; 
for  on  the  following  morning  they  were  still  slightl}^ 
green.  The  same  result  followed,  even  still  more 
clearly,  in  two  cases  with  leaflets  of  Melilotus  officinalis 
which  projected  a  little  beyond  the  cork ;  and  in  two 
other  cases  some  leaflets  which  were  pinned  close  to 
the  cork  were  injured,  whilst  other  free  leaflets  on 
the  same  leaves,  which  had  not  space  to  rotate  and 
assume  their  proper  vertical  position,  were  not  at  all 
injured. 

Another  analogous  fact  deserves  notice  :  we  observed 
on  several  occasions  that  a  greater  number  of  free 
leaves  were  injured  on  the  branches  which  had  been 
kept  motionless  by  some  of  their  leaves  having  been 
pinned  to  the  corks,  than  on  the  other  branches.  This 
was  conspicuously  the  case  with  those  of  Melilotus 
Pettynerreana,  but  the  injured  leaves  in  this  instance 
were  not  actually  counted.  With  Arachis  hypogwa,  a 
young  plant  with  7  stems  bore  22  free  leaves,  and  of 
these  5  were  injured  by  the  frost,  all  of  which  were  on 
two  stems,  bearing  four  leaves  pinned  to  the  cork- 
supports.  With  Oxalis  carnosa,  7  free  leaves  were 
injured,  and  every  one  of  them  belonged  to  a  cluster 
of  leaves,  some  of  which  had  been  pinned  to  the  cork. 
We  could  account  for  these  cases  only  by  supposing 
that  the  branches  which  were  quite  free  had  been 
slightly  waved  about  by  the  wind,  and  that  their 
leaves  had  thus  been  a  little  warmed  by  the  sur- 
rounding warmer  air.  If  we  hold  our  hands  motion- 
before  a  hot  fire,  and  then  wave  them  about,  we 


CHAP.  VI.  SLEEP  OF  COTYLEDONS.  297 

immediately  feel  relief;  and  this  is  evidently  an 
analogous,  though  reversed,  case.  These  several  facts 
—  in  relation  to  leaves  pinned  close  to  or  a  little  above 
the  cork-supports — to  their  tips  projecting  beyond  it — 
and  to  the  leaves  on  branches  kept  motionless — seem 
to  us  curious,  as  showing  how  a  difference,  apparently 
trifling,  may  determine  the  greater  or  less  injury  of 
the  leaves.  We  may  even  infer  as  probable  that  the 
less  or  greater  destruction  during  a  frost  of  the  leaves 
on  a  plant  which  does  not  sleep,  may  often  depend  on 
the  greater  or  less  degree  of  flexibility  of  their  petioles 
and  of  the  branches  which  bear  them. 

NYCTITKOPIC  OK  SLEEP  MOVEMENTS  OF  COTYLEDONS. 

We  now  come  to  the  descriptive  part  of  our  work, 
and  will  begin  with  cotyledons,  passing  on  to  leaves 
in  the  next  chapter.  We  have  met  with  only  two 
brief  notices  of  cotyledons  sleeping.  Hofmeister,* 
after  stating  that  the  cotyledons  of  all  the  observed 
seedlings  of  the  Caryophyllese  (Alsinese  and  Silenese) 
bend  upwards  at  night  (but  to  what  angle  he  does  not 
state),  remarks  that  those  of  Stellaria  media  rise  up  so 
as  to  touch  one  another ;  they  may  therefore  safely  be 
said  to  sleep.  Secondly,  according  to  Eamey,|  the 
cotyledons  of  Mimosa  pudica  and  of  CUanthus  Dam- 
pieri  rise  up  almost  vertically  at  night  and  approach 
each  other  closely.  It  has  been  shown  in  a  previous 
chapter  that  the  cotyledons  of  a  large  number  of 
plants  bend  a  little  upwards  at  night,  and  we  here 
have  to  meet  the  difficult  question  at  what  inclination 
may  they  be  said  to  sleep?  According  to  the  view 
which  we  maintain,  no  movement  deserves  to  be  called 


*  *  Die  Lehre  von  der  Pflanzenzelle,'  1867,  p.  327. 
t  *  Adansonia,'  March  10th,  1869. 


298  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

nyctitropic,  unless  it  has  been  acquired  for  the  sake  of 
lessening  radiation ;  but  this  could  be  discovered  only 
by  a  long  series  of  experiments,  showing  that  '.the 
leaves  of  each  species  suffered  from  this  cause,  if  pre- 
vented from  sleeping.  We  must  therefore  take  an 
arbitrary  limit.  If  a  cotyledon  or  leaf  is  inclined  at 
60°  above  or  beneath  the  horizon,  it  exposes  to  the 
zenith  about  one-half  of  its  area;  consequently  the 
intensity  of  its  radiation  will  be  lessened  by  about 
half,  compared  with  what  it  would  have  been  if  the 
cotyledon  or  leaf  had  remained  horizontal.  This 
degree  of  diminution  certainly  would  make  a  great 
difference  to  a  plant  having  a  tender  constitution. 
We  will  therefore  speak  of  a  cotyledon  and  hereafter 
of  a  leaf  as  sleeping,  only  when  it  rises  at  night  to 
an  angle  of  about  60°,  or  to  a  still  higher  angle,  above 
the  horizon,  or  sinks  beneath  it  to  the  same  amount. 
Not  but  that  a  lesser  diminution  of  radiation  may  be 
advantageous  to  a  plant,  as  in  the  case  of  Datura 
stramonium,  the  cotyledons  of  which  rose  from  31°  at 
noon  to  55°  at  night  above  the  horizon.  The  Swedish 
turnip  may  profit  by  the  area  of  its  leaves  being 
reduced  at  night  by  about  30  per  cent,,  as  estimated 
by  ~M  r.  A.  S.  Wilson ;  though  in  this  case  the  angle 
through  which  the  leaves  rose  was  not  observed.  On 
the  other  hand,  when  the  angular  rise  of  cotyledons  or 
of  leaves  is  small,  such  as  less  than  30°,  the  diminution 
of  radiation  is  so  slight  that  it  probably  is  of  no  sig- 
nificance to  the  plant  in  relation  to  radiation.  For 
instance,  the  cotyledons  of  Geranium  Ilericum  rose  at 
night  to  27°  above  the  horizon,  and  this  would  lessen 
radiation  by  only  11  per  cent. :  those  of  Linum  Beren- 
dieri  rose  to  33°,  and  this  would  lessen  radiation  by 
16  per  cent. 

There  are,  however,  some  other  sources  of  doubt  with 


CHAP.  VI.  SLEEP   OF   COTYLEDONS.  299 

respect  to  the  sleep  of  cotyledons.  In  certain  cases, 
the  cotyledons  whilst  young  diverge  during  the  day  to 
only  a  very  moderate  extent,  so  that  a  small  rise  at 
night,  which  we  know  occurs  with  the  cotyledons  of 
many  plants,  would  necessarily  cause  them  to  assume 
a  vertical  or  nearly  vertical  position  at  night ;  and  in 
this  case  it  would  be  rash  to  infer  that  the  movement 
was  effected  for  any  special  purpose.  On  this  account 
we  hesitated  long  whether  we  should  introduce  several 
Cucurbitaceous  plants  into  the  following  list ;  but  from 
reasons,  presently  to  be  given,  we  thought  that  they 
had  better  be  at  least  temporarily  included.  This 
same  source  of  doubt  applies  in  some  few  other  cases ; 
for  at  the  commencement  of  our  observations  we  did 
not  always  attend  sufficiently  to  whether  the  cotyle- 
dons stood  nearly  horizontally  in  the  middle  of  the  day. 
With  several  seedlings,  the  cotyledons  assume  a  highly 
inclined  position  at  night  during  so  short  a  period  of 
their  life,  that  a  doubt  naturally  arises  whether  this 
can  be  of  any  service  to  the  plant.  Nevertheless,  in 
most  of  the  cases  given  in  the  following  list,  the  coty- 
ledons may  be  as  certainly  said  to  sleep  as  may  the 
leaves  of  any  plant.  In  two  cases,  namely,  with  the 
cabbage  and  radish,  the  cotyledons  of  which  rise  almost 
vertically  during  the  few  first  nights  of  their  life,  it 
was  ascertained  by  placing  young  seedlings  in  the 
klinostat,  that  the  upward  movement  was  not  due  to 
apogeotropism. 

The  names  of  the  plants,  the  cotyledons  of  which 
stand  at  night  at  an  angle  of  at  least  60°  with  the 
horizon,  are  arranged  in  the  appended  list  on  the  same 
system  as  previously  followed.  The  numbers  of  the 
Families,  and  with  the  Leguminosae  the  numbers  of 
the  Tribes,  have  been  added  to  show  how  widely 
the  plants  in  question  are  distributed  throughout  the 


300 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VI. 


dicotyledonous  series.  A  few  remarks  will  have  to 
be  made  about  many  of  the  plants  in  the  list.  In 
doing  so,  it  will  be  convenient  not  to  follow  strictly 
any  systematic  order,  but  to  treat  of  the  Oxalidse 
and  the  Leguminosae  at  the  close ;  for  in  these 
two  Families  the  cotyledons  are  generally  provided 
with  a  pulvinus,  and  their  movements  endure  for  a 
much  longer  time  than  those  of  the  other  plants  in 
the  list. 


List  of  Seedling  Plants,  the  cotyledons  of  which  rise  or  sink  at 
night  to  an  angle  of  at  least  60°  above  or  beneath  the  horizon. 


Brassica  oleracea.  Cruciferse  (Fam. 
14). 

napus  (as  we  are  informed 

by  Prof.  Pfefier). 

Raphaiius  sativus.     Cruciferse. 

Githago  segetum.  Caryophylleae 
(Fam.  26). 

Stellaria  media  (according  to  Hof- 
meister,  as  quoted).  Caryophyl- 
leae. 

Anoda  Wrightii.  Malvaceae  (Fam. 
36). 

Gossypium  (var.  Nankin  cotton). 
Malvaceae. 

Oxalis  rosea.     Oxalidse  (Fam,  41). 

floribunda. 

articulata. 

Valdiviana. 

sensitiva.  • 

Geranium  rotundifolium.  Gera- 
niaceae  (Fam.  47). 

Trifolium  subterraneum.  Legu- 
minosae (Fam.  75,  Tribe  3). 

strictum. 

leucanthemum. 

Lotus  ornithopopoides.  Legumi- 
nosae (Tribe  4). 

peregrinus. 

Jacobaeus. 

Clianthus  Dampieri.  Legumi- 
nosae (Tribe  5)— according  to  M. 
Ramey. 

Smithia  sensitiva.  Leguminosae 
(Tribe  6). 

Hsematoxvlon  Campechianum.    Le- 


guminosae (Tribe  13) — accord- 
ing to  Mr.  R.  I.  Lynch. 

Cassia  mimosoides.  Leguminosre 
(Tribe  14). 

glauca. 

lloruhi. 

corymbosa. 

pubescens. 

tora. 

neglecta. 

3  other  Brazilian  unnamed 

species. 
Bauhinia    (sp.    ?).        Leguminosae 

(Tribe  15> 
Neptunia     oleracea.      Leguminosae 

(Tribe  20). 
Mimosa       pudica.         Leguminosie 

(Tribe  21). 
albida. 


Cucurbitacese 


Cucurbitaceae. 

Cucurbitaceae. 

Umbelliferae 


Cucurbita   ovifera. 

(Fam.  1D6). 

aurantia. 

Lagenaria  vulgaris. 
Cucumis  dudaim. 
Apium  petroselinum. 

(Fam.  113). 

graveolens. 

Lactuca  scariola.    Compositae  (Fam. 

122). 

Helianthus  animus  (?).    Compositae. 
Ipomoea    caerulea.      Convolvulacea 

(Fam.  151). 

pur  p  urea. 

bona-noi. 


CHAP.  VI.  SLEEP  OF   COTYLEDONS.  301 

List  of  Seedling  Plants  (continued}. 

Solanum    lycopersicum.       Solanese 

(Fam.  157). 
Mimulus,    (sp.  ?)    Scrophularinese 

(Fam.    159)  —  from   information 

given  us  by  Prof.  Pfeffer. 
Mirabilis      jalapa.          Nyctaginese 


Mirabilis  longiflcra. 

Beta    vulgaris.     Polygonese    (Fam. 

179). 
Amaranthus    caudatus.      Amaran- 

thacese  (Fam.  180). 
Cannabis    sativa  (?).      Cannabineae 


(Fam.  177).  (Fam.  195). 

Brassica  oleracea  (Cruciferse).  —It  was  shown  in  the  first  chapter 
that  the  cotyledons  of  the  common  cabbage  rise  in  the  evening 
and  stand  vertically  up  at  night  with  their  petioles  in  contact. 
But  as  the  two  cotyledons  are  of  unequal  height,  they  frequently 
interfere  a  little  with  each  other's  movements,  the  shorter  one 
often  not  standing  quite  vertically.  They  awake  early  in  the 
morning ;  thus  at  6.45  A.M.  on  Nov.  27th,  whilst  it  was  still 
dark,  the  cotyledons,  which  had  been  vertical  and  in  contact  on 
the  previous  evening,  were  reflexed,  and  thus  presented  a  very 
different  appearance.  It  should  be  borne  in  mind  that  seedlings 
in  germinating  at  the  proper  season,  would  not  be  subjected  to 
darkness  at  this  hour  in  the  morning.  The  above  amount  of 
movement  of  the  cotyledons  is  only  temporary,  lasting  with  plants 
kept  in  a  warm  greenhouse  from  four  to  six  days ;  how  long  it 
would  last  with  seedlings  growing  out  of  doors  we  do  not  know. 

Haphanus  sativus. — In  the  middle  of  the  day  the  blades  of 
the  cotyledons  of  10  seedlings  stood  at  right  angles  to  their 
hypocotyls,  with  their  petioles  a  little  divergent ;  at  night  the 
blades  stood  vertically,  with  their  bases  in  contact  and  with 
their  petioles  parallel.  Next  morning,  at  6.45A.M.,  whilst  it 
was  still  dark,  the  blades  were  horizontal.  On  the  following 
night  they  were  much  raised,  but  hardly  stood  sufficiently  ver- 
tical to  be  said  to  be  asleep,  and  so  it  was  in  a  still  less  degree 
on  the  third  night.  Therefore  the  cotyledons  of  this  plant  (kept 
in  the  greenhouse)  go  to  sleep  for  even  a  shorter  time  than 
those  of  the  cabbage.  Similar  observations  were  made,  but  only 
during  a  single  day  and  night,  on  13  other  seedlings  likewise 
raised  in  the  greenhouse,  with  the  same  result. 

The  petioles  of  the  cotyledons  of  11  young  seedlings  of 
Sinapis  nigra  were  slightly  divergent  at  noon,  and  the  blades 
stood  at  right  angles  to  the  hypocotyls ;  at  night  the  petioles 
were  in  close  contact,  and  the  blades  considerably  raised, 
with  their  bases  in  contact,  but  only  a  few  stood  sufficiently 
upright  to  be  called  asleep.  On  the  following  morning, 


302  MODIFIED  CIRCUMNUTATION.  CIIAI-.  VI. 

the  petioles  diverged  before  it  was  light.  The  hypocotyl  is 
slightly  sensitive,  so  that  if  rubbed  with  a  needle  it  bends 
towards  the  rubbed  side.  In  the  case  of  Lepidium  mtivum,  the 
petioles  of  the  cotyledons  of  young  seedlings  diverge  during 
the  day  and  converge  so  as  to  touch  each  other  during  the 
night,  by  which  means  the  bases  of  the  tripartite  blades  are 
brought  into  contact ;  but  the  blades  are  so  little  raised  that 
they  cannot  be  said  to  sleep.  The  cotyledons  of  several  other 
cruciferous  plants  were  observed,  but  they  did  not  rise  sufficiently 
during  the  night  to  be  said  to  sleep. 

Githago  segetum  (Caryophyllese). — On  the  first  day  after  the 
cotyledons  had  burst  through  the  seed-coats,  they  stood  at  noon 
at  an  angle  of  75°  above  the  horizon ;  at  night  they  moved 
upwards,  each  through  an  angle  of  15°  so  as  to  stand  quite 
vertical  and  in  contact  with  one  another.  On  the  second  day 
they  stood  at  noon  at  59°  above  the  horizon,  and  again  at 
night  were  completely  closed,  each  having  risen  31°.  On  the 
fourth  day  the  cotyledons  did  not  quite  close  at  night.  The 
first  and  succeeding  pairs  of  young  true  leaves  behaved  in 
exactly  the  same  manner.  We  think  that  the  movement  in  this 
case  may  be  called  nyctitropic,  though  the  angle  passed  through 
was  small.  The  cotyledons  are  very  sensitive  to  light  and  will 
not  expand  if  exposed  to  an  extremely  dim  one. 

Anoda  Wrightii  (Malvaceae). — The  cotyledons  whilst  moderately 
young,  and  only  from  '2  to  *3  inch  in  diameter,  sink  in  the 
evening  from  their  mid-day  horizontal  position  to  about  35° 
beneath  the  horizon.  But  when  the  same  seedlings  were  older 
and  had  produced  small  true  leaves,  the  almost  orbicular 
cotyledons,  now  *55  inch  in  diameter,  moved  vertically  downwards 
at  night.  This  fact  made  us  suspect  that  their  sinking  might 
be  due  merely  to  their  weight ;  but  they  were  not  in  the  least 
flaccid,  and  when  lifted  up  sprang  back  through  elasticity  into 
their  former  dependent  position.  A  pot  with  some  old  seedlings 
was  turned  upside  down  in  the  afternoon,  before  the  noc- 
turnal fall  had  commenced,  and  at  night  they  assumed  in  op- 
position to  their  own  weight  (and  to  any  geotropic  action)  an 
upwardly  directed  vertical  position.  When  pots  were  thus 
reversed,  after  the  evening  fall  had  already  commenced,  the 
sinking  movement  appeared  to  be  somewhat  disturbed ;  but  all 
their  movements  were  occasionally  variable  without  any  apparent 
cause.  This  latter  fact,  as  well  as  that  of  the  young  cotyledons 
not  sinking  nearly  so  much  as  the  older  ones,  deserves  notice. 


CHAP.  VI.  SLEEP   OF   COTYLEDONS.  303 

Although  the  movement  of  the  cotyledons  endured  for  a  long 
time,  no  pulvinus  was  exteriorly  visible;  but  their  growth 
continued  for  a  long  time.  The  cotyledons  appear  to  be  only 
slightly  heliotropic,  though  the  hypocotyl  is  strongly  so. 

Gossypium  arboreum  (?)  (var.  Nankin  cotton)  (Malvaceae). — The 
cotyledons  behave  in  nearly  the  same  manner  as  those  of  the 
Anoda.  On  June  15th  the  cotyledons  of  two  seedlings  were 
•65  inch  in  length  (measured  along  the  midrib)  and  stood  hori- 
zontally at  noon ;  at  10  P.M.  they  occupied  the  same  position 
and  had  not  fallen  at  all.  On  June  23rd,  the  cotyledons  of  one 
of  these  seedlings  were  I'l  inch  in  length,  and  by  10  P.M.  they 
had  fallen  from  a  horizontal  position  to  62°  beneath  the  horizon. 
The  cotyledons  of  the  other  seedling  were  1*3  inch  in  length,  and 
a  minute  true  leaf  had  been  formed ;  they  had  fallen  at  10  P.M. 
to  70°  beneath  the  horizon.  On  June  25th,  the  true  leaf  of  this 
latter  seedling  was  '9  inch  in  length,  and  the  cotyledons  occu- 
pied nearly  the  same  position  at  night.  By  July  9th  the  cotyle- 
dons appeared  very  old  and  showed  signs  of  withering;  but  they 
stood  at  noon  almost  horizontally,  and  at  10  P.M.  hung  down 
vertically. 

Oossypium  herbaceum. — It  is  remarkable  that  the  cotyledons  of 
this  species  behave  differently  from  those  of  the  last.  They  were 
observed  during  6  weeks  from  their  first  development  until 
they  had  grown  to  a  very  large  size  (still  appearing  fresh  and 
green),  viz.  2 £  inches  in  breadth.  At  this  age  a  true  leaf  had 
been  formed,  which  with  its  petiole  was  2  inches  long.  During 
the  whole  of  these  6  weeks  the  cotyledons  did  not  sink  at  night ; 
yet  when  old  their  weight  was  considerable  and  they  were  borne 
by  much  elongated  petioles.  Seedlings  raised  from  some  seed 
sent  us  from  Naples,  behaved  in  the  same  manner ;  as  did  those 
of  a  kind  cultivated  in  Alabama  and  of  the  Sea-island  cotton. 
To  what  species  these  three  latter  forms  belong  we  do  not  know. 
We  could  not  make  out  in  the  case  of  the  Naples  cotton,  that 
the  position  of  the  cotyledons  at  night  was  influenced  by  the 
soil  being  more  or  less  dry ;  care  being  taken  that  they  were 
not  rendered  flaccid  by  being  too  dry.  The  weight  of  the  large 
cotyledons  of  the  Alabama  and  Sea-island  kinds  caused  them  to 
hang  somewhat  downwards,  when  the  pots  in  which  they  grew 
were  left  for  a  time  upside  down.  It  should,  however,  be 
observed  that  these  three  kinds  were  raised  in  the  middle  of 
the  winter,  which  sometimes  greatly  interferes  with  the  proper 
nyctitropic  movements  of  leaves  and  cotyledons. 
14 


304  MODIFIED   CIRCUMXUTATION.  CHAP.  VI. 

Cucurbit*  icece. — The  cotyledons  of  Cueurlita  aurantia  and  ovi- 
fera,  and  of  Lagenaria  vulgar  is,  stand  from  the  1st  to  the  3rd  day 
of  their  life  at  about  60°  above  the  horizon,  and  at  night  rise  up 
so  as  to  become  vertical  and  in  close  contact  with  one  another. 
With  Cucumis  dudaim,  they  stood  at  noon  at  45°  above  the  hori- 
zon, and  closed  at  night.  The  tips  of  the  cotyledons  of  all  these 
species  are,  however,  reflexed,  so  that  this  part  is  fully  exposed 
to  the  zenith  at  night ;  and  this  fact  is  opposed  to  the  belief 
that  the  movement  is  of  the  same  nature  as  that  of  sleeping 
plants.  After  the  first  two  or  three  days  the  cotyledons 
diverge  more  during  the  day  and  cease  to  close  at  night. 
Those  of  Trichosanthes  anguina  are  somewhat  thick  and  fleshy, 
and  did  not  rise  at  night ;  and  they  could  perhaps  hardly  be 
expected  to  do  so.  On  the  other  hand,  those  of  Acanthosicyos 
horrida  *  present  nothing  in  their  appearance  opposed  to  their 
moving  at  night  in  the  same  manner  as  the  preceding  species  ; 
yet  they  did  not  rise  up  in  any  plain  manner.  This  fact  leads 
to  the  belief  that  the  nocturnal  movements  of  the  above-named 
species  has  been  acquired  for  some  special  purpose,  which  may 
be  to  protect  the  young  plumule  from  radiation,  by  the  close 
contact  of  the  whole  basal  portion  of  the  two  cotyledons. 

Gtranium  rotundifolium  (Geraniacere).— A  single  seedling  came 
up  accidentally  in  a  pot,  and  its  cotyledons  were  observed  to 
bend  perpendicularly  downwards  during  several  successive 
nights,  having  been  horizontal  at  noon.  It  grew  into  a  fine 
plant  but  died  before  flowering :  it  was  sent  to  Kew  and  pro- 
nounced to  be  certainly  a  Geranium,  and  in  all  probability  the 
above-named  species.  This  case  is  remarkable  because  the 
cotyledons  of  G.  cinereum,  Endressii,  Ibericum,  Bichardsoni,  and 
subcaukscens  were  observed  during  some  weeks  in  the  winter, 
and  they  did  not  sink,  whilst  those  of  G.  Jb  ricum  rose  27°  at 
night. 

Apiuiin  petroselinum  (Umbelliferse). — A  seedling  had  its  coty- 
ledons (Nov.  22nd)  almost  fully  expanded  during  the  day ;  by 
8.30  P.M.  they  had  risen  considerably,  and  at  10.30  P.M.  were 
almost  closed,  their  tips  being  only  ^  of  an  inch  apart.  On 
the  following  morning  (23rd)  the  tips  were  T%8o  of  an  inch  apart, 


*  This  plant,   from  Dammara  climber;   it  has  been    described 

Land  in  S.Africa,  is  remarkable  in    'Transact.  Linn.  Soc.,'  xxvii 

from  being  the  one  known  mem-  p.  30. 
her  of  the  Family  which  is  not  a 


CHAP.  VI.  SLEEP  OF   COTYLEDONS.  305 

or  more  than  seven  times  as  much.  On  the  next  night  the 
cotyledons  occupied  nearly  the  same  position  as  before.  On  the 
morning  of  the  21th  they  stood  horizontally,  and  at  night  were 
60°  above  the  horizon ;  and  so  it  was  on  the  night  of  the  25th. 
But  four  days  afterwards  (on  the  29th),  when  the  seedlings 
were  a  week  old,  the  cotyledons  had  ceased  to  rise  at  night  to 
any  plain  degree. 

Apium  graveolens. — The  cotyledons  at  noon  were  horizontal, 
and  at  10  P.M.  stood  at  an  angle  of  61°  above  the  horizon. 

Lactuca  scariola  (Composite) . — The  cotyledons  whilst  young 
stood  sub-horizontally  during  the  day,  and  at  night  rose  so  as 
to  be  almost  vertical,  and  some  were  quite  vertical  and  closed ; 
but  this  movement  ceased  when  they  had  grown  old  and  large, 
after  an  interval  of  11  days. 

Helianthus  annum  (Compositae). — This  case  is  rather  doubtful ; 
the  cotyledons  rise  at  night,  and  on  one  occasion  they  stood  at 
73°  above  the  horizon,  so  that  they  might  then  be  said  to  have 
been  asleep. 

Ipomcea  ccerulea  vel  Pharbitis  nil  (Convolvulacese). — The  coty- 
ledons behave  in  nearly  the  same  manner  as  those  of  the  Anoda 
and  Nankin  cotton,  and  like  them  grow  to  a  large  size.  Whilst 
young  and  small,  so  that  their  blades  were  from  '5  to  '6  of  an 
inch  in  length,  measured  along  the  middle  to  the  base  of  the 
central  notch,  they  remained  horizontal  both  during  the  middle 
of  the  day  and  at  night.  As  they  increased  in  size  they  began 
to  sink  more  and  more  in  the  evening  and  early  night ;  and 
when  they  had  grown  to  a  length  (measured  in  the  above 
manner)  of  from  J  to  T25  inch,  they  sank  between  55°  and  70° 
beneath  the  horizon.  They  acted,  however,  in  this  manner  only 
when  they  had  been  well  illuminated  during  the  day.  Never- 
theless, the  cotyledons  have  little  or  no  power  of  bending 
towards  a  lateral  light,  although  the  hypocotyl  is  strongly  helio- 
tropic.  They  are  not  provided  with  a  pulvinus,  but  continue 
to  grow  for  a  long  time. 

Ipomoea  purpurea  (vel  Pharbitis  hispida). — The  cotyledons 
behave  in  all  respects  like  those  of  /.  ccerulea.  A  seedling  with 
cotyledons  '75  inch  in  length  (measured  as  before)  and  rt>5 
inch  in  breadth,  having  a  small  true  leaf  developed,  was  placed 
at  5.30  P.M.  on  a  klinostat  in  a  darkened  box,  so  that  neither 
weight  nor  geotropism  could  act  on  them.  At  10  P.M.  one  coty- 
ledon stood  at  77°  and  the  other  at  82°  beneath  the  horizon. 
Before  being  placed  in  the  klinostat  they  stood  at  15°  and  2(JC 


306  MODIFIED   CIKCUMXUTATION.  CHAP.  VI 

beneath  the  horizon.  The  nocturnal  position  depends  chiefly 
on  the  curvature  of  the  petiole  close  to  the  blade,  but  the  whole 
petiole  becomes  slightly  curved  downwards.  It  deserves  notice 
that  seedlings  of  this  and  the  last-named  species  were  raised  at 
the  end  of  February  and  another  lot  in  the  middle  of  March, 
and  the  cotyledons  in  neither  case  exhibited  any  nyctitropic 
movement. 

Jpomaa  bona-nox. — The  cotyledons  after  a  few  days  grow  to 
an  enormous  size,  those  on  a  young  seedling  being  3i  inches 
in  breadth.  They  were  extended  horizontally  at  noon,  and  at 
10  P.M.  stood  at  63°  beneath  the  horizon.  Five  days  after- 
wards they  were  4*  inches  in  breadth,  and  at  night  one  stood  at 
64°  and  the  other  48°  beneath  the  horizon.  Though  the  blades 
are  thin,  yet  from  their  great  size  and  from  the  petioles  being 
long,  we  imagined  that  their  depression  at  night  might  be 
determined  by  their  weight ;  but  when  the  pot  was  laid  hori- 
zontally, they  became  curved  towards  the  hypocotyl,  which 
movement  could  not  have  been  in  the  least  aided  by  their 
weight,  at  the  same  time  they  were  somewhat  twisted  upwards 
through  apogeotropism.  Nevertheless,  the  weight  of  the  coty- 
ledons is  so  far  influential,  that  when  on  another  night  the  pot 
was  turned  upside  down,  they  were  unable  to  rise  and  thus  to 
assume  their  proper  nocturnal  position. 

Jpom'ia  coccinea. — The  cotyledons  whilst  young  do  not  sink 
at  night,  but  when  grown  a  little  older,  but  still  only  '4  inch  in 
length  (measured  as  before)  and  -82  in  breadth,  they  became 
greatly  depressed.  In  one  case  they  were  horizontal  at  noon, 
and  at  10  P.M.  one  of  them  stood  at  64°  and  the  other  at  47° 
beneath  the  horizon.  The  blades  are  thin,  and  the  petioles, 
which  become  much  curved  down  at  night,  are  short,  so  that 
here  weight  can  hardly  have  produced  any  effect.  With  all  the 
above  species  of  Ipomcea,  when  the  two  cotyledons  on  the  same 
seedling  were  unequally  depressed  at  night,  this  seemed  to 
depend  on  the  position  which  they  had  held  during  the  day 
with  reference  to  the  light. 

Solatium  lycopersicum  (Solanese). —  The  cotyledons  rise  so 
much  at  night  as  to  come  nearly  in  contact.  Those  of  S.  palina- 
canthum  were  horizontal  at  noon,  and  by  10  P.M.  had  risen  only 
27°  30' ;  but  on  the  following  morning  before  it  was  light  they 
stood  at  59°  above  the  horizon,  and  in  the  afternoon  of  the  same 
day  were  again  horizontal.  The  behaviour  of  the  cotyledons  of 
this  latter  species  seems,  therefore,  to  be  anomalous. 


CHAP.  VI.  SLEEP   OF   COTYLEDONS.  307 

Mirabilis  jalapa  and  lonyiflora  (Nyctagineae). — The  cotyledons, 
which  are  of  unequal  size,  stand  horizontally  during  the  middle 
of  the  day,  and  at  night  rise  up  vertically  and  come  into  close 
contact  with  one  another.  But  this  movement  with  M.  longijlori 
lasted  for  only  the  three  first  nights. 

Btta,  vulgaris  (Polygonese). — A  large  number  of  seedlings  were 
observed  on  three  occasions.  During  the  day  the  cotyledons 
sometimes  stood  sub-horizontally,  but  more  commonly  at  an 
angle  of  about  50°  aboye  the  horizon,  and  for  the  first  two  or 
three  nights  they  rose  up  vertically  so  as  to  be  completely 
closed.  During  the  succeeding  one  or  two  nights  they  rose 
only  a  little,  and  afterwards  hardly  at  all. 

Amaranthus  caudatus  (Amaranthacese). — At  noon  the  coty- 
ledons of  many  seedlings,  which  had  just  germinated,  stood  at 
about  45°  above  the  horizon,  and  at  10.15  P.M.  some  were  nearly 
and  others  quite  closed.  On  the  following  morning  they  were 
again  well  expanded  or  open. 

Cannabis  sativa  (Cannabineae). — We  are  very  doubtful  whether 
this  plant  ought  to  be  here  included.  The  cotyledons  of  a  large 
number  of  seedlings,  after  being  well  illuminated  during  the 
day,  were  curved  downwards  at  night,  so  that  the  tips  of  some 
pointed  directly  to  the  ground,  but  the  basal  part  did  not  appear 
to  be  at  all  depressed.  On  the  following  morning  they  were 
again  flat  and  horizontal.  The  cotyledons  of  many  other  seed- 
lings were  at  the  same  time  not  in  any  way  affected.  Therefore 
this  case  seems  very  different  from  that  of  ordinary  sleep,  and' 
probably  comes  under  the  head  of  epinasty,  as  is  the  case  with 
the  leaves  of  this  plant  according  to  Kraus.  The  cotyledons  are 
heliotropic,  and  so  is  the  hypocotyl  in  a  still  stronger  degree. 

Oxalis. — We  now  come  to  cotyledons  provided  with  a  pulvinus, 
all  of  which  are  remarkable  from  the  continuance  of  the  nocturnal 
movements  during  several  days  or  even  weeks,  and  apparently 
after  growth  has  ceased.  The  cotyledons  of  0.  rosea,floribunda 
and  articulate  sink  vertically  down  at  night  and  clasp  the  upper 
part  of  the  hypocotyl.  Those  of  0.  Valdiviana  and  scnsitiva,,  on 
the  contrary,  rise  vertically  up,  so  that  their  upper  surfaces  come 
into  close  contact ;  and  after  the  young  leaves  are  developed  these 
are  clasped  by  the  cotyledons.  As  in  the  daytime  they  stand  hori- 
zontally, or  are  even  a  little  deflected  beneath  the  horizon,  they 
move  in  the  evening  through  an  angle  of  at  least  90°.  Their 
complicated  circumnutating  movements  during  the  day  havd 


308  MODIFIED  CIRCUMNUTATION.  CHAP.  VI 

been  described  in  the  first  chapter.  The  experiment  was  a 
superfluous  one,  but  pots  with  seedlings  of  0.  rosea  and  floribunda 
were  turned  upside  down,  as  soon  as  the  cotyledons  began  to 
show  any  signs  of  sleep,  and  this  made  no  difference  in  their 
movements. 

Leguminosce. — It  may  be  seen  in  our  list  that  the  cotyledons 
of  several  species  in  nine  genera,  widely  distributed  through- 
out the  Family,  sleep  at  night ;  and  this  probably  is  the  case 
with  many  others.  The  cotyledons  of  £,11  these  species  are  pro- 
vided with  a  pulvinus ;  and  the  movement  in  all  is  continued 
during  many  days  or  weeks.  In  Cassia  the  cotyledons  of  the 
ten  species  in  the  list  rise  up  vertically  at  night  and  come 
into  close  contact  with  one  another.  We  observed  that  those 
of  (7.  florida  opened  in  the  morning  rather  later  than  those  of 
C.  glaum  and  pubescens.  The  movement  is  exactly  the  same 
in  C.  mimosoides  as  in  the  other  species,  though  its  subsequently 
developed  leaves  sleep  in  a  different  manner.  The  cotyledons 
of  an  eleventh  species,  namely,  C.  nodosa,  are  thick  and  fleshy, 
and  do  not  rise  up  at  night.  The  circumnutation  of  the  coty- 
ledons during  the  day  of  C.  tora  has  been  described  in  the  first 
chapter.  Although  the  cotyledons  of  Smithia  sensitiva  rose  from 
a  horizontal  position  in  the  middle  of  the  day  to  a  vertical  one 
at  night,  those  of  S.  Pfundii,  which  are  thick  and  fleshy,  did  not 
sleep.  When  Mimosa  pudica  and  albida  have  been  kept  at  a 
sufficiently  high  temperature  during  the  day,  the  cotyledons 
corne  into  close  contact  at  night ;  otherwise  they  merely  rise  up 
almost  vertically.  The  circumnutation  of  those  -of  M.  pudica 
has  been  described.  The  cotyledons  of  a  Bauhinia  from  St. 
Catharina  in  Brazil  stood  during  the  day  at  an  angle  of  about 
5'j°  above  the  horizon,  and  at  night  rose  to  77°;  but  it  is  pro- 
bable that  they  would  have  closed  completely,  if  the  seedlings 
had  been  kept  in  a  warmer  place. 

Lotus. — In  three  species  of  Lotus  the  cotyledons  were  observed 
to  sleep.  Those  of  L.  Jacobceus  present  the  singular  case  of  not 
rising  at  night  in  any  conspicuous  manner  for  the  first  5  or 
6  days  of  their  life,  and  the  pulvinus  is  not  well  developed  at 
this  period.  Afterwards  the  sleeping  movement  is  well  dis- 
played, though  to  a  variable  degree,  and  is  long  continued. 
We  shall  hereafter  meet  with  a  nearly  parallel  case  with  the 
leaves  of  Sida  rhombifulia.  The  cotyledons  of  L.  Gebelii  are 
only  slightly  raised  at  night,  and  differ  much  in  this  respect 
from  the  three  species  in  our  list. 


CHAP.  VI.  SLEEP  OF   COTYLEDONS.  309 

Trifolium. — The  germination  of  21  species  was  observed.  In 
most  of  them  the  cotyledons  rise  hardly  at  all,  or  only  slightly, 
at  night ;  but  those  of  T.  glomeratum,  striatum  and  incarnatum 
rose  from  45°  to  55°  above  the  horizon.  With  T.  subterraneum, 
leucanthemum  and  strictum,  they  stood  up  vertically ;  and  with 
T.  strictum  the  rising  movement  is  accompanied,  as  we  shall  see, 
by  another  movement,  which  makes  us  believe  that  the  rising 
is  truly  nyctitropic.  We  did  not  carefully  examine  the  coty- 
ledons of  all  the  species  for  a  pulvinus,  but  this  organ  was 
distinctly  present  in  those  of  T.  subterraneum  and  strictum ;  whilst 
there  was  no  trace  of  a  pulvinus  in  some  species,  for  instance,  in 
T.  resupinatum,  the  cotyledons  of  which  do  not  rise  at  night. 

Trifolium  subterraneum. — The  blades  of  the  cotyledons  on  the 
first  day  after  germination  (Nov.  21st)  were  not  fully  expanded, 
being  inclined  at  about  35°  above  the  horizon ;  at  night  they 
rose  to  about  75°.  Two  days  afterwards  the  blades  at  noon 
were  horizontal,  with  the  petioles  highly  inclined  upwards; 
and  it  is  remarkable  that  the  nocturnal  movement  is  almost 
wholly  confined  to  the  blades,  being  effected  by  the  pulvinus  at 
their  bases ;  whilst  the  petioles  retain  day  and  night  nearly  the 
same  inclination.  On  this  night  (Nov.  23rd),  and  for  some  few 
succeeding  nights,  the  blades  rose  from  a  horizontal  into  a 
vertical  position,  and  then  became  bowed  inwards  at  about  an 
average  angle  of  10° ;  so  that  they  had  passed  through  an  angle 
of  100°,  Their  tips  now  almost  touched  one  another,  their 
bases  being  .slightly  divergent.  The  two  blades  thus  formed 
a  highly  inclined  roof  over  the  axis  of  the  seedling.  This 
movement  is  the  same  as  that  of  the  terminal  leaflet  of  the 
tripartite  leaves  of  many  species  of  Trifolium.  After  an  interval 
of  8  days  (Nov.  29th)  the  blades  were  horizontal  during  the 
day,  and  vertical  at  night,  and  now  they  were  no  longer  bowed 
inwards.  They  continued  to  move  in  the  same  manner  for  the 
following  two  months,  by  which  time  they  had  increased  greatly 
in  size,  their  petioles  being  no  less  than  '8  of  an  inch  in  length, 
and  two  true  leaves  had  by  this  time  been  developed. 

Trifolium  strictum. — On  the  first  day  after  germination  the 
cotyledons,  which  are  provided  with  a  pulvinus,  stood  at  noon 
horizontally,  and  at  night  rose  to  only  about  45°  above  the 
horizon.  Four  days  afterwards  the  seedlings  were  again  ob- 
served at  night,  and  now  the  blades  stood  vertically  and  were 
in  contact,  excepting  the  tips,  which  were  much  deflexed,  so 
that  they  faced  the  zenith.  At  this  age  the  petioles  are  curved 


olO  MODIFIED   CIRCUMNUTATION.  CIIAP.  VI. 

upwards,  and  at  night,  when  the  bases  of  the  blades  are  in  con- 
tact, the  two  petioles  together  form  a  vertical  ring  surrounding 
the  plumule.  The  cotyledons  continued  to  act  in  nearly  the  same 
manner  for  8  or  10  days  from  the  period  of  germination ;  but 
the  petioles  had  by  this  time  become  straight  and  had  increased 
much  in  length.  After  from  12  to  14  days  the  first  simple  true 
leaf  was  formed,  and  during  the  ensuing  fortnight  a  remarkable 
movement  was  repeatedly  observed.  At  I.  (Fig.  125)  we  have 
a  sketch,  made  in  the  middle  of  the  day,  of  a  seedling  about 
a  fortnight  old.  The  two  cotyledons,  of  which  Re  is  the 
light,  and  Lc  the  left  one,  stand  directly  opposite  one  another, 


I.  II.  III. 


Trifoliu'n  strictum :  diurnal  and  nocturnal  positions  of  the  two  cotyledon  > 
and  of  the  first  leaf.  I.  Seedling  viewed  obliquely  from  above,  during 
the  day :  Re,  right  cotyledon ;  Lc,  left  cotyledon ;  F,  first  true  leaf. 
II.  A  rather  younger  seedling,  viewed  at  night:  7?c,  right  cotyledon 
raised,  but  its  position  not  otherwise  changed  ;  Lc,  left  cotyledon  raised 
and  laterally  twisted ;  F,  first  leaf  raised  and  twisted  so  .as  to  face  the 
left  twisted  cotyledon.  III.  Same  seedling  viewed  at  night  from  the 
opposite  side.  The  back  of  the  first  leaf,  F,  is  here  shown  instead  of 
the  front,  as  in  II. 

and  the  first  true  leaf  (F}  projects  at  right  angles  to  them.  At 
night  (see  II.  and  III.)  the  right  cotyledon  (Be)  is  greatly 
raised,  but  is  not  otherwise  changed  in  position.  The  left 
cotyledon  (Lc)  is  likewise  raised,  but  it  is  also  twisted,  so  that 
its  blade,  instead  of  exactly  facing  the  opposite  one,  now  stands 
at  nearly  right  angles  to  it  This  nocturnal  twisting  movement 
is  effected  not  by  means  of  the  pulvinus,  but  by  the  twisting  of 
the  whole  length  of  the  petiole,  as  could  be  seen  by  the  curved 
Jine  of  its  upper  concave  surface.  At  the  same  time  the  true 
leaf  (F)  rises  up,  so  as  to  stand  vertically,  or  it  even  passes  the 
vertical  and  is  inclined  a  little  inwards.  It  also  twists  a  little, 
by  which  means  the  upper  surface  of  its  blade  fronts,  and 
almost  comes  into  contact  with,  the  upper  surface  of  the  twisted 


CHAP.  VI.  SLEEP  OF   COTYLEDONS.  311 

left  cotyledon.  This  seems  to  be  the  object  gained  by  these 
singular  movements.  Altogether  20  seedlings  were  examined  on 
successive  nights,  and  in  19  of  them  it  was  the  left  cotyledon 
alone  which  became  twisted,  with  the  true  leaf  always  so  twisted 
that  its  upper  surface  approached  closely  and  fronted  that  of  the 
left  cotyledon.  In  only  one  instance  was  the  right  cotyledon 
twisted,  with  the  true  leaf  twisted  towards  it ;  but  this  seedling 
was  in  an  abnormal  condition,  as  the  left  cotyledon  did  not  rise 
up  properly  at  night.  This  whole  case  is  remarkable,  as  with 
the  cotyledons  of  no  other  plant  have  we  seen  any  nocturnal 
movement  except  vertically  upwards  or  downwards.  It  is  the 
more  remarkable,  because  we  shall  meet  with  an  analogous  case 
in  the  leaves  of  the  allied  genus  Melilotus,  in  which  the  ter- 
minal leaflet  rotates  at  night  so  as  to  present  one  edge  to  the 
zenith  and  at  the  same  time  bends  to  one  side,  so  that  its  upper 
surface  comes  into  contact  with  that  of  one  of  the  two  now  ver- 
tical lateral  leaflets. 

Concluding  Remarks  on  the  Nijditropic  Movements  o/ 
Cotyledons. — The  sleep  of  cotyledons  (though  this  is  a 
subject  which  has  been  little  attended  to),  seems  to  be 
a  more  common  phenomenon  than  that  of  leaves.  We 
observed  the  position  of  the  cotyledons  during  the  day 
and  night  in  153  genera,  widely  distributed  through- 
out the  dicotyledonous  series,  but  otherwise  selected 
almost  by  hazard;  and  one  or  more  species  in  26  of 
these  genera  placed  their  cotyledons  at  night  so  as 
to  stand  vertically  or  almost  vertically,  having  gene- 
rally moved  through  an  angle  of  at  least  60°.  If  we 
lay  on  one  side  the  Leguminosae,  the  cotyledons  of 
which  are  particularly  liable  to  sleep,  140  genera 
remain  ;  and  out  of  these,  the  cotyledons  of  at  least  one 
species  in  19  genera  slept.  Now  if  we  were  to  select 
by  hazard  140  genera,  excluding  the  Leguminosse,  and 
observed  their  leaves  at  night,  assuredly  not  nearly 
so  many  as  19  would  be  found  to  include  sleeping 
species.  We  here  refer  exclusively  to  the  plants 
observed  by  ourselves. 


312  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

In  our  entire  list  of  seedlings,  there  are  30  genera, 
belonging  to  16  Families,  the  cotyledons  of  which  in 
some  of  the  species  rise  or  sink  in  the  evening  or 
early  night,  so  as  to  stand  at  least  60°  above  or  be- 
neath the  horizon.  In  a  large  majority  of  the  genera, 
namely,  24,  the  movement  is  a  rising  one;  so  that 
the  same  direction  prevails  in  these  nyctitropic  move- 
ments as  in  the  lesser  periodic  ones  described  in  the 
second  chapter.  The  cotyledons  move  downwards 
during  the  early  part  of  the  night  in  only  6  of  the 
genera;  and  in  one  of  them,  Cannabis,  the  curving 
down  of  the  tip  is  probably  due  to  epinasty,  as  Kraus 
believes  to  be  the  case  with  the  leaves.  The  down- 
ward movement  to  the  amount  of  90°  is  very  decided 
in  Oxalis  Valdiviana  and  sensitiva,  and  in  Geranium 
rotundifolium.  It  is  a  remarkable  fact  that  with  Anoda 
Wriglitii,  one  species  of  Gossypiurn  and  at  least  3 
species  of  Ipomoea,  the  cotyledons  whilst  young  and 
light  sink  at  night  very  little  or  not  at  all ;  although 
this  movement  becomes  well  pronounced  as  soon  as 
they  have  grown  large  and  heavy.  Although  the 
downward  movement  cannot  be  attributed  to  the 
weight  of  the  cotyledons  in  the  several  cases  which 
were  investigated,  namely,  in  those  of  the  Anoda, 
Ipomoea  purpurea  and  bona-nox,  nor  in  that  of  I  COG- 
cinea,  yet  bearing  in  mind  that  cotyledons  are  con- 
tinually circumnutating,  a  slight  cause  might  at  first 
have  determined  whether  the  great  nocturnal  move- 
ment should  be  upwards  or  downwards.  We  may 
therefore  suspect  that  in  some  aboriginal  member  of 
the  groups  in  question,  the  weight  of  the  cotyledons 
first  determined  the  downward  direction.  The  fact  of 
the  cotyledons  of  these  species  not  sinking  down  much 
whilst  they  are  young  and  tender,  seems  opposed  to 
the  belief  that  the  greater  movement  when  they  are 


CHAP.  VI.  SLEEP   OF   COTYLEDONS.  313 

grown  older,  has  been  acquired  for  the  sake  of  pro- 
tecting them  from  radiation  at  night ;  but  then  we 
should  remember  that  there  are  many  plants,  the 
leaves  of  which  sleep,  whilst  the  cotyledons  do  not ; 
and  if  in  some  cases  the  leaves  are  protected  from  cold 
at  night  whilst  the  cotyledons  are  not  protected,  so  in 
other  cases  it  may  be  of  more  importance  to  the  species 
that  the  nearly  full-grown  cotyledons  should  be  better 
protected  than  the  young  ones. 

In  all  the  species  of  Oxalis  observed  by  us,  the  coty- 
ledons are  provided  with  pulvini ;  but  this  organ  has 
become  more  or  less  rudimentary  in  0.  corniculata, 
and  the  amount  of  upward  movement  of  its  cotyledons 
at  night  is  very  variable,  but  is  never  enough  to  be 
called  sleep.  We  omitted  to  ascertain  whether  the 
cotyledons  of  Geranium  rotundifolium  possess  pulvini. 
In  the  Leguminos£e  all  the  cotyledons  which  sleep,  as 
far  as  we  have  seen,  are  provided  with  pulvini.  But 
with  Lotus  Jacobseus,  these  are  not  fully  developed 
during  the  first  few  days  of  the  life  of  the  seedling, 
and  the  cotyledons  do  not  then  rise  much  at  night. 
With  Trifolium  strictum  the  blades  of  the  cotyledons 
rise  at  night  by  the  aid  of  their  pulvini;  whilst  the 
petiole  of  one  cotyledon  twists  half-round  at  the  same 
time,  independently  of  its  pulvinus. 

As  a  general  rule,  cotyledons  which  are  provided 
with  pulvini  continue  to  rise  or  sink  at  night  during 
a  much  longer  period  than  those  destitute  of  this  organ. 
In  this  latter  case  the  movement  no  doubt  depends  on 
alternately  greater  growth  on  the  upper  and  lower  side 
of  the  petiole,  or  of  the  blade,  or  of  both,  preceded 
probably  by  the  increased  turgescence  of  the  growing 
cells.  Such  movements  generally  last  for  a  very 
short  period — for  instance,  with  Brassica  and  Grithago 
for  4  or  5  nights,  with  Beta  for  2  or  3,  and  with 


314  MODIFIED   CIRCUMNUTATION.  CHAP.  VI. 

Kaphanus  for  only  a  single  night.  There  are,  however, 
some  strong  exceptions  to  this  rule,  as  the  cotyledons 
of  Gossypium,  Anoda  and  Ipomoaa  do  not  possess  pul- 
vini,  yet  continue  to  move  and  to  grow  for  a  long  time. 
We  thought  at  first  that  when  the  movement  lasted  for 
only  2  or  3  nights,  it  could  hardly  be  of  any  service 
to  the  plant,  and  hardly  deserved  to  be  called  sleep ; 
but  as  many  quickly-growing  leaves  sleep  for  only  a 
few  nights,  and  as  cotyledons  are  rapidly  developed 
and  soon  complete  their  growth,  this  doubt  now  seems 
to  us  not  well-founded,  more  especially  as  these  move- 
ments are  in  many  instances  so  strongly  pronounced. 
We  may  here  mention  another  point  of  similarity 
between  sleeping  leaves  and  cotyledons,  namely,  that 
some  of  the  latter  (for  instance,  those  of  Cassia  and 
Githago)  are  easily  affected  by  the  absence  of  light ; 
and  they  then  either  close,  or  if  closed  do  not  open ; 
whereas  others  (as  with  the  cotyledons  of  Oxalis)  are 
very  little  affected  by  light.  In  the  next  chapter  it 
will  be  shown  that  the  nyctitropic  movements  both 
of  cotyledons  and  leaves  consist  of  a  modified  form  of 
circumnutation. 

As  in  the  Leguminosse  and  Oxalidse,  the  leaves  and 
the  cotyledons  of  the  same  species  generally  sleep,  the 
idea  at  first  naturally  occurred  to  us,  that  the  sleep 
of  the  cotyledons  was  merely  an  early  development  of 
a  habit  proper  to  a  more  advanced  stage  of  life.  But 
no  such  explanation  can  be  admitted,  although  there 
seems  to  be  some  connection,  as  might  have  been 
expected,  between  the  two  sets  of  cases.  For  the 
leaves  of  many  plants  sleep,  whilst  their  cotyledons  do 
not  do  so — of  which  fact  Desmodium  gyrans  offers  a 
good  instance,  as  likewise  do  three  species  of  Nico- 
tiana  observed  by  us ;  also  Sida  rhombifolia,  Abutilon 
Darwinii,  and  Chenopodium  album.  On  the  other 


CHAP.  VI.  SLEEP  OF  COTYLEDONS.  315 

hand,  the  cotyledons  of  some  plants  sleep  and  not  the 
leaves,  as  with  the  species  of  Beta,  Brassica,  Geranium, 
Apium,  Solanum,  and  Mirabilis,  named  in  our  list. 
Still  more  striking  is  the  fact  that,  in  the  same  genus, 
the  leaves  of  several  or  of  all  the  species  may  sleep, 
but  the  cotyledons  of  only  some  of  them,  as  occurs 
with  Trifolium,  Lotus,  Gossypium,  and  partially  with 
Oxalis.  Again,  when  both  the  cotyledons  and  the 
leaves  of  the  same  plant  sleep,  their  movements  may 
be  of  a  widely  dissimilar  nature  :  thus  with  Cassia  the 
cotyledons  rise  vertically  up  at  night,  whilst  their 
leaves  sink  down  and  twist  round  so  as  to  turn  their 
lower  surfaces  outwards.  With  seedlings  of  Oxalis 
Valdiviana,  having  2  or  3  well-developed  leaves,  it 
was  a  curious  spectacle  to  behold  at  night  each  leaflet 
folded  inwards  and  hanging  perpendicularly  down- 
wards, whilst  at  the  same  time  and  on  the  same  plant 
the  cotyledons  stood  vertically  upwards. 

These  several  facts,  showing  the  independence  of 
the  nocturnal  movements  of  the  leaves  and  cotyledons 
on  the  same  plant,  and  on  plants  belonging  to  the 
same  genus,  lead  to  the  belief  that  the  cotyledons  have 
acquired  their  power  of  movement  for  some  special 
purpose.  Other  facts  lead  to  the  same  conclusion, 
such  as  the  presence  of  pulvini,  by  the  aid  of  which 
the  nocturnal  movement  is  continued  during  some 
weeks.  In  Oxalis  the  cotyledons  of  some  species 
move  vertically  upwards,  and  of  others  vertically 
downwards  at  night ;  but  this  great  difference  within 
the  same  natural  genus  is  not  so  surprising  as  it 
may  at  first  appear,  seeing  that  the  cotyledons  of  all 
the  species  are  continually  oscillating  up  and  down 
during  the  day,  so  that  a  small  cause  might  determine 
whether  they  should  rise  or  sink  at  night.  Again,  the 
peculiar  nocturnal  movement  of  the  left-hand  coty- 


316  MODIFIED  CIRCUMNUTATION.  CHAP.  VI. 

ledon  of  Trifolium  strictum,  in  combination  with  that 
of  the  first  true  leaf.  Lastly,  the  wide  distribution  in 
the  dicotyledonous  series  of  plants  with  cotyledons 
which  sleep.  Reflecting  on  these  several  facts,  our 
conclusion  seems  justified,  that  the  nycti tropic  move- 
ments of  cotyledons,  by  which  the  blade  is  made  to 
stand  either  vertically  or  almost  vertically  upwards 
or  downwards  at  night,  has  been  acquired,  at  least 
in  most  cases,  for  some  special  purpose ;  nor  can  we 
doubt  that  this  purpose  is  the  protection  of  the  upper 
surface  of  the  blade,  and  perhaps  of  the  central  bud 
or  plumule,  from  radiation  at  night. 


CHAP.  VIL  MODIFIED   CIRCUMNUTATION.  317 


CHAPTER  VIL 

MODIFIED  CIRCUMNUTATION  :  NYCTITROPIC  OR  SLEEP  MOVEMENTS  OP 
LEAVES. 

Conditions  necessary  for  these  movements — List  of  Genera  and  Families, 
which  include  sleeping  plants — Description  of  the  movements  in 
the  several  Genera — Oxalis :  leaflets  folded  at  night — Averrhoa  : 
rapid  movements  of  the  leaflets — Porlieria:  leaflets  close  when 
plant  kept  very  dry — Tropseolum :  leaves  do  not  sleep  unless  well 
illuminated  during  day — Lupinus:  various  modes  of  sleeping — 
Melilotus  :  singular  movements  of  terminal  leaflet — Trifolium — 
Desmodium:  rudimentary  lateral  leaflets,  movements  of,  not  de- 
veloped on  young  plants,  state  of  their  pulvini — Cassia :  complex 
movements  of  the  leaflets — Bauhinia:  leaves  folded  at  night — 
Mimosa  pudica :  compounded  movements  of  leaves,  effect  of  dark- 
ness— Mimosa  albida,  reduced  leaflets  of — Schrankia:  downward 
movement  of  the  pinnae — Marsilea  :  the  only  cryptogam  known  to 
sleep — Concluding  remarks  and  summary — Nyctitropisrn  consists 
of  modified  circumnutation,  regulated  by  the  alternations  of  light 
and  darkness — Shape  of  first  true  leaves. 

WE  now  come  to  the  iiyctitropic  or  sleep  move- 
ments of  leaves.  It  should  be  remembered  that  we 
confine  this  term  to  leaves  which  place  their  blades 
at  night  either  in  a  vertical  position  or  not  more  than 
30°  from  the  vertical, — that  is,  at  least  60°  above  or 
beneath  the  horizon.  In  some  few  cases  this  is 
effected  by  the  rotation  of  the  blade,  the  petiole  not 
being  either  raised  or  lowered  to  any  considerable 
extent.  The  limit  of  30°  from  the  vertical  is  obviously 
an  arbitrary  one,  and  has  been  selected  for  reasons 
previously  assigned,  namely,  that  when  the  blade 
approaches  the  perpendicular  as  nearly  as  this,  only 
half  as  much  of  the  surface  is  exposed  at  night  to  the 


318  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

zenith  and  to  free  radiation  as  when  the  blade  is 
horizontal.  Nevertheless,  in  a  few  instances,  leaves 
which  seem  to  be  prevented  by  their  structure  from 
moving  to  so  great  an  extent  as  60°  above  or  beneath 
the  horizon,  have  been  included  amongst  sleeping 
plants. 

It  should  be  premised  that  the  nyctitropic  move- 
ments of  leaves  are  easily  affected  by  the  conditions 
to  which  the  plants  have  been  subjected.  If  the  ground 
is  kept  too  dry,  the  movements  are  much  delayed 
or  fail :  according  to  Dassen,*  even  if  the  air  is 
very  dry  the  leaves  of  Impatiens  and  Malva  are 
rendered  motionless.  Carl  Kraus  has  also  lately 
insisted  f  on  the  great  influence  which  the  quantity  of 
water  absorbed  has  on  the  periodic  movements  of 
leaves ;  and  he  believes  that  this  cause  chiefly  deter- 
mines the  variable  amount  of  sinking  of  the  leaves  of 
Polygonum  convolvulus  at  night ;  and  if  so,  their  move- 
ments are  not  in  our  sense  strictly  nyctitropic.  Plants 
in  order  to  sleep  must  have  been  exposed  to  a  proper 
temperature :  Erythrina  crista-galli,  out  of  doors  and 
nailed  against  a  wall,  seemed  in  fairly  good  health, 
but  the  leaflets  did  not  sleep,  whilst  those  on  another 
plant  kept  in  a  warm  greenhouse  were  all  vertically  de- 
pendent at  night.  In  a  kitchen-garden  the  leaflets  of 
Phaseolus  vulgaris  did  not  sleep  during  the  early  part 
of  the  summer.  Ch.  Koyer  says,|  referring  I  suppose 
to  the  native  plants  in  France,  that  they  do  not  sleep 
when  the  temperature  is  below  5°  C.  or  41°  F.  In 
the  case  of  several  sleeping  plants,  viz.,  species  of 


*  Dassen,  '  Tijdschrift  vor.  Na-  Bot.'  (5th  series^),  ix.  1868,  p.  345. 

turlijke   Gesch.   en   Physiologie,'  f  '  Beitrage    zur  Kentuiss  der 

1837,  vol.  iv.  p.  106.      See  also  Bewcgungcn,'    &c.,    in     'Flora,' 

Ch.  Royer  on  the  importance  of  a  1879,  pp.  42,  43,  67,  &o. 

proper  state  of  turgescence  of  the  J  '  Annal.   des  Sc.   Nat.  Bot.' 

cells,   in  •  Annal.   des   Sc.   Nat.  (5th  Series),  ix.   1868  p.366. 


CHAP.  VII.  SLEEP   OF   LEAVES.  ,     319 

Tropaeolum,  Lupinus,  Ipomoea,  Abutilon,  Siegesbeckia, 
and  probably  other  genera,  it  is  indispensable  that 
the  leaves  should  be  well  illuminated  during  the  day 
in  order  that  they  may  assume  at  night  a  vertical 
position ;  and  it  was  probably  owing  to  this  cause 
that  seedlings  of  Chenopodium  album  and  Siegesbeckia 
orientalis,  raised  by  us  during  the  middle  of  the  winter, 
though  kept  at  a  proper  temperature,  did  not  sleep. 
Lastly,  violent  agitation  by  a  strong  wind,  during  a 
few  minutes,  of  the  leaves  of  Maranta  arundinacea 
(which  previously  had  not  been  disturbed  in  the  hot- 
house), prevented  their  sleeping  during  the  two  next 
nights. 

We  will  now  give  our  observations  on  sleeping 
plants,  made  in  the  manner  described  in  the  Intro- 
duction. The  stem  of  the  plant  was  always  secured 
(when  not  stated  to  the  contrary)  close  to  the  base  of 
the  leaf,  the  movements  of  which  were  being  observed, 
so  as  to  prevent  the  stem  from  circumnutating.  As 
the  tracings  were  made  on  a  vertical  glass  in  front  of 
the  plant,  it  was  obviously  impossible  to  trace  its 
course  as  soon  as  the  leaf  became  in  the  evening 
greatly  inclined  either  upwards  or  downwards;  it 
must  therefore  be  understood  that  the  broken  lines 
in  the  diagrams,  which  represent  the  evening  and 
nocturnal  courses,  ought  always  to  be  prolonged  to  a 
much  greater  distance,  either  upwards  or  downwards, 
than  appears  in  them.  The  conclusions  which  may  be 
deduced  from  our  observations  will  be  given  near  the 
end  of  this  chapter. 

In  the  following  list  all  the  genera  which  include 
sleeping  plants  are  given,  as  far  as  known  to  us.  The 
same  arrangement  is  followed  as  in  former  cases,  and 
the  number  of  the  Family  is  appended.  This  list 
possesses  some  interest,  as  it  shows  that  the  habit  of 


320 


MODIFIED  CIRCUMXUTATION. 


CHAP.  VII. 


sleeping  is  common  to  some  few  plants  throughout 
the  whole  vascular  series.  The  greater  number  of  the 
genera  in  the  list  have  been  observed  by  ourselves 
with  more  or  less  care ;  but  several  are  given  on  the 
authority  of  others  (whose  names  are  appended  in  the 
list),  and  about  these  we  have  nothing  more  to  say. 
No  doubt  the  list  is  very  imperfect,  and  several  genera 
might  have  been  added  from  the  l  Somnus  Plantarum' 
by  Linnaeus ;  but  we  could  not  judge,  in  some  of  his 
cases,  whether  the  blades  occupied  at  night  a  nearly 
vertical  position.  He  refers  to  some  plants  as  sleeping, 
for  instance,  Latliyrus  odoratus  and  Vicia  faba,  in  which 
we  could  observe  no  movement  deserving  to  be  called 
sleep,  and  as  no  one  can  doubt  the  accuracy  of  LinnseuSj 
we  are  left  in  doubt. 

List  of  Genera,  including  species  the  leaves  of  winch  sleep. 


CLASS  I.  DICOTYLEDONS. 

Sub-class  I.  ANGIOSPERMS  —  continued. 

Sub-class  I.  . 
Genus. 
Githago 

&.NGIOSPERMS. 

Family. 
Caryophylleae  (26). 

Genus. 
Tropaeolum. 
Crotolaria  (Thisel-\ 
ton  Dyer).          J 

Family. 
Tropaeoleae  (49). 
Leguminosae      (75) 
Tribe  II. 

Stellaria  (Batalin). 

5> 

Lupinus. 

,,             ,, 

Portulaca        (Ch.\ 
Royer).               / 

Portulaceae  (27). 

Cytisus. 
Trigonella. 

n                >» 

„       Tr.  III. 

Sida. 

Malvaceae  (36). 

Medicago. 

'9                      " 

Abutilon. 

,» 

Melilotus. 

Malva      (Linnaeus'i 

Tri  folium. 

and  Pfeffer).      / 

>» 

Securigera. 

"        Tr.'lV. 

Hibiscus         (Lin-"l 

Lotus. 

??                        9? 

naeus).                 / 

» 

Psoralea. 

Tr.  V. 

Anoda. 

» 

Amorpha        (Du-'i 

Gossypium. 

,J 

chartre).             / 

?J                          ?> 

Ayenia  (Linnaeus). 

Sterculaceje  (37). 

Daalea. 

3>                       99 

Triumfetta    (Lin-1 
naeus).                 / 

Tiliaceaa  (38). 

Indigofera. 
Tephrosia. 

99                        99 
99                        99 

Linum  (Batalin). 

Lineae  (39). 

Wistaria. 

99                       99 

Oxalis. 

Oxalidae  (41). 

Robinia. 

99                       99 

Averrhoa. 

»» 

Sphaerophysa. 

99                        99 

Porlieria. 

Zygophylleae  (45). 

Colutea. 

99                        99 

Guiacum. 

,, 

Astragalus. 

«t                       t« 

Impatiens       (Lin-J 

Glycyrrhiza.                       „             „ 

naeus,       Pfeffer,  > 

BaJsamineae  (48). 

Coronilla.                           „       Tr.  VI. 

Batalin).             J 

j  Hcdysarum.                       „            „ 

CHAP.  VII.  SLEEP  OF  LEAVES. 


List  of  Genera  (continued}. 


321 


CLASS  I.  DICOTYLEDONS  (continued). 

Sub-class  I.  ANGUOSPERMS  (continued). 

Sub-class  I.  ANGIOSPERMS. 

Genus. 

Fami'y. 

Genus. 

Family. 

./Enothera       (Lin-1 
naeus).              •   / 

Onagrarieae  (100). 

Onobrychis. 

JLeguminosae    (75) 
1       „       Tr.  VI. 

Passiflora. 
Siegesbeckia. 

Passifloraceae(105)< 
Composite  (122). 

Smithia. 

11             11 

JConvolvulaceas 

Arachis. 

11             11 

Ipomoea. 

\     051). 

Desmodium. 

11             11 

Nicotiana. 

Solaneae  (157). 

Urania. 

11             11 

Mirabilis. 

Nyctagineaj  (177). 

Vicia. 
Centrosema. 

,       Tr.  VII. 
,       Tr.  VIII. 

Polygonum     (Ba-1 
talin).                 / 

Polygonese  (179). 

Amphicarpsea. 
Glycine. 

, 

Amaranth  us. 

jAmaranthaceae 
I     (180). 

Erythrina. 

,            „ 

Chenopodium. 

Chenopodieae  (181). 

Apios. 

i            j> 

Pimelia  (Bouchd). 

Thvmeteae  (188). 

Phaseolus. 

i            11 

Euphorbia. 

Euphorbiaceae(202) 

Sophora. 
Cicsalpinia. 

,        Tr.  X. 
,      Tr.  XIII. 

Phyllanthus(Pfef-'l 

11 

Hoematoxylon. 

i             11 

Gleditsclna     (Du-j 

Sub-class  11.  GYM^OSPERMS. 

chartre).             / 
Poinciana. 

" 

Abies  (Chatin). 

Cassia. 

Tr.  XIV. 

Bauhinia. 

„       Tr.  XV. 

CLASS  II.  MONOCOTYLEDONS. 

Tamarindu?. 

„       Tr.  XVI. 

Adenanthera. 

„       Tr.  XX. 

Thalia. 

Cannaceae  (21). 

Prosopis. 

Maranta. 

„ 

Neptunia. 

"             " 

Colocasia. 

Aroideac  (30). 

Mimosa. 

11             11 

Strephium. 

Gramineie  (55). 

Schrankia. 

»             11 

Acacia. 
Albizzia. 

„     Tr.XXII. 
„     Tr.  XXIII. 

CLASS  III.  ACOTYLEDONS. 

Melaleuca  (Bouche). 

Myrtaceae  (94). 

Marsilea.                      Marsileaceae  (4). 

Githago  segetum  (Caryophyllere). — The  first  leaves  produced 
by  young  seedlings,  rise  up  and  close  together  at  night.  On  a 
rather  older  seedling,  two  young  leaves  stood  at  noon  at  55° 
above  the  horizon,  and  at  night  at  86°,  so  each  had  risen  31°. 
The  angle,  however,  was  less  in  some  cases.  Similar  observations 
were  occasionally  made  on  young  leaves  (for  the  older  ones  moved 
very  little)  produced  by  nearly  full-grown  plants.  Batalin 
says  ('  Flora/  Oct.  1st,  1873,  p.  437)  that  the  young  leaves  of 
Stellaria  close  up  so  completely  at  night  that  they  form  together 
great  buds. 

Sida  (Malvaceae). — The  nyctitropic  movements  of  the  leaves 
in  this  genus  are  remarkable  in  some  respects.  Batalin  informs 


322 


MODIFIED   CIRCUMXUTATION.          CHAP.  VII. 


Fig.  126 


us  (see  also  'Flora,'  Oct.   1st,   1873,  p.   437)   that   those   ot 

S.  napcea  fall  at  night,  but 
to  what  angle  he  cannot 
remember.  The  leaves  of 
S.  rhombifolia  and  retusa,  on 
the  other  hand,  rise  up 
vertically,  and  are  pressed 
against  the  stem.  We  have 
therefore  here  within  the 
same  genus,  directly  op- 
posite movements.  Again, 
the  leaves  of  S.  rJwmbifolia 
are  furnished  with  a  pul- 
vinus, formed  of  a  mass  of 
small  cells  destitute  of  chlo- 
rophyll, and  with  their 
longer  axes  perpendicular 
to  the  axis  of  the  petiole. 
As  measured  along  this 
latter  line,  these  cells  are 
only  ith  of  the  length  of 
those  of  the  petiole;  but 
instead  of  being  abruptly 
separated  from  them  (as  is 
usual  with  the  pulvinus  in 
most  plants),  they  graduate 
into  the  larger  cells,  of  the 
petiole.  On  the  other  hand, 
S.  napcea,  according  to  Ba- 
talin,  does  not  possess  a 
pulvinus;  and  he  informs 
us  that  a  gradation  may  be 
traced  in  the  several  species 
S,.da,  rhombifolia  :  circumnutation  and  of  tne  genus  between  these 
nyctitropic  (or  sleep)  movements  of  two  states  of  the  petiole, 
a  leaf  on  a  young  plant,  9 }  inches  $ida  rhombifolia  presents 
high;  filament  fixed  to  midrib  of  ,,  ,.  ..  .  ,.  <• 

newly  full-grown  leaf,  2|  inches  in  another  peculiarity,  of  which 
length  ;  movement  traced  under  a  sky-  We  have  seen  no  other  in- 
light.  Apex  of  leaf  5|  inches  from  stance  with  leaves  that 

sleep :    for  those   on    very 
young  plants,  though  they 
rise  somewhat  in  the  evening,  do  not  go  to  sleep,  as  we  observed 


CHAP.  VII.  SLEEP   OF   LEAVES.  323 

on  several  occasions ;  whilst  those  on  rather  older  plants  sleep 
in  a  conspicuous  manner.  For  instance,  a  leaf  (-85  of  an  inch 
in  length)  on  a  very  young  seedling  2  inches  high,  stood  at  noon 
9°  above  the  horizon,  and  at  10  P.M.  at  28°,  so  it  had  risen  only 
19°;  another  leaf  (1'4  inch  in  length)  on  a  seedling  of  the 
same  height,  stood  at  the  same  two  periods  at  7°  and  32°,  and 
therefore  had  risen  25°.  These  leaves,  which  moved  so  little, 
had  a  fairly  well-developed  pulvinus.  After  an  interval  of  some 
weeks,  when  the  same  seedlings  were  2|  and  3  inches  in  height, 
some  of  the  young  leaves  stood  up  at  night  quite  vertically,  and 
others  were  highly  inclined ;  and  so  it  was  with  bushes  which 
were  fully  grown  and  were  flowering. 

The  movement  of  a  leaf  was  traced  from  9.15  A.M.  on 
May  28th  to  8.30  A.M.  on  the  30th.  The  temperature  was  too 
low  (15° — 16°  C.),  and  the  illumination  hardly  sufficient;  con- 
sequently the  leaves  did  not  become  quite  so  highly  inclined  at 
night,  as  they  had  done  previously  and  as  they  did  subse- 
quently in  the  hot-house ;  but  the  movements  did  not  appear 
otherwise  disturbed.  On  the  first  day  the  leaf  sank  till 
5.15  P.M.  ;  it  then  rose  rapidly  and  greatly  till  10.5  P.M.,  and 
only  a  little  higher  during  the  rest  of  the  night  (Fig.  126). 
Early  on  the  next  day  (29th)  it  fell  in  a  slightly  zigzag  line 
rapidly  until  9  A.M.,  by  which  time  it  had  reached  nearly  the 
same  place  as  on  the  previous  morning.  During  the  remainder 
of  the  day  it  fell  slowly,  and  zigzagged  laterally.  The  evening 
rise  began  after  4  P.M.  in  the  same  manner  as  before,  arid  on 
the  second  morning  it  again  fell  rapidly.  The  ascending  and 
descending  lines  do  not  coincide,  as  may  be  seen  in  the  diagram. 
On  the  30th  a  new  tracing  was  made  (not  here  given)  on  a 
rather  enlarged  scale,  as  the  apex  of  the  leaf  now  stood  9  inches 
from  the  vertical  glass.  In  order  to  observe  more  carefully  the 
course  pursued  at  the  time  when  the  diurnal  fall  changes  into 
the  nocturnal  rise,  dots  were  made  every  half-hour  between 
4  P.M.  and  10.30  P.M.  This  rendered  the  lateral  zigzagging 
movement  during  the  evening  more  conspicuous  than  in  the 
diagram  given,  but  it  was  of  the  same  nature  as  there  shown. 
The  impression  forced  on  our  minds  was  that  the  leaf  was 
expending  superfluous  movement,  so  that  the  great  nocturnal 
rise  might  not  occur  at  too  early  an  hour. 

Alutilon  Darwinii  (Malvaceae). — The  leaves  on  some  very 
young  plants  stood  almost  horizontally  during  the  day,  and 
hung  down  vertically  at  night.  Very  fine  plants  kept  in  a 


324  MODIFIED  CIRCUMNUTATION.          CHAP.  VII. 

large  hall,  lighted  only  from  the  roof,  did  not  sleep  at  night, 
for  in  order  to  do  so  the  leaves  must  be  well  illuminated  during 
the  day.  The  cotyledons  do  not  sleep.  Linnaeus  says  that  the 
leaves  of  his  Sid<i  abutilon  sink  perpendicularly  down  at  night, 
though  the  petioles  rise.  Prof.  Pfeffer  informs  us  that  the 
leaves  of  a  Malva,  allied  to  M.  sylvestris,  rise  greatly  at  night ; 
and  this  genus,  as  well  as  that  of  Hibiscus,  are  included  by 
Linnaeus  in  his  list  of  sleeping  plants. 

Anoda  Wrightii  (Malvaceae). — The  leaves,  produced  by  very 
young  plants,  when  grown  to  a  moderate  size,  sink  at  night 
either  almost  vertically  down  or  to  an  angle  of  about  45°  beneath 
the  horizon ;  for  there  is  a  considerable  degree  of  variability  in 
the  amount  of  sinking  at  night,  which  depends  in  part  on  the 
degree  to  which  they  have  been  illuminated  during  the  day. 
But  the  leaves,  whilst  quite  young,  do  not  sink  down  at  night, 
and  this  is  a  very  unusual  circumstance.  The  summit  of  the 
petiole,  where  it  joins  the  blade,  is  developed  into  a  pulvinus, 
and  this  is  present  in  very  young  leaves  which  do  not  sleep ; 
though  it  is  not  so  well  defined  as  in  older  leaves. 

Qossypium  (var.  Nankin  cotton,  Malvaceae). — Some  young 
leaves,  between  1  and  2  inches  in  length,  borne  by  two  seedlings 
6  and  7  2  inches  in  height,  stood  horizontally,  or  were  raised  a 
little  above  the  horizon  at  noon  on  July  8th  and  9th ;  but  by 
10  P.M.  they  had  sunk  down  to  between  68°  and  90°  beneath 
the  horizon.  When  the  same  plants  had  grown  to  double 
the  above  height,  their  leaves  stood  at  night  almost  or  quite 
vertically  dependent.  The  leaves  on  some  large  plants  of 
G.  maritimum  and  Brazilense,  which  were  kept  in  a  very  badly 
lighted  hot-house,  only  occasionally  sank  much  downwards 
at  night,  and  hardly  enough  to  be  called  sleep. 

Oxalis  (Oxalidse). — In  most  of  the  species  in  this  large  genus 
the  three  leaflets  sink  vertically  down  at  night;  but  as  their 
sub-petioles  are  short  the  blades  could  not  assume  this  position 
from  the  want  of  space,  unless  they  were  in  some  manner  ren- 
dered narrower;  and  this  is  effected  by  their  becoming  more 
or  less  folded  (Fig.  127).  The  angle  formed  by  the  two  halves 
of  the  same  leaflet  was  found  to  vary  in  different  individuals  of 
several  species  between  92°  and  150°;  in  three  of  the  best 
folded  leaflets  of  0.  fragrans  it  was  76°,  74°,  and  54°.  The 
angle  is  often  different  in  the  three  leaflets  of  the  same  leaf. 
As  the  leaflets  sink  down  at  night  and  become  folded,  their 
lower  surfaces  are  brought  near  together  (see  B),  or  even  into 


CHAP.  VII.  SLEEP   OF   LEAVES.  325 

close  contact ;  and  from  this  circumstance  it  might  be  thought 
that  the  object  of  the  folding  was  the  protection  of  their  lower 
surfaces.  If  this  had  been  the  case,  it  would  have  formed 
a  strongly  marked  exception  to  the  rule,  that  when  there  is  any 
difference  in  the  degree  of  protection  from  radiation  of  the  two 
surfaces  of  the  leaves,  it  is  always  the  upper  surface  which  is 
the  best  protected.  But  that  the  folding  of  the  leaflets,  and 
consequent  mutual  approximation  of  their  lower  surfaces, 
serves  merely  to  allow  them  to  sink  down  vertically,  may  be 

Fig.  127. 


A.  B. 

Oxalis  acetoseUa:  A,  leaf  seen  from  vertically  above  ;  B,  diagram  of  leaf 
asleep,  also  seen  from  vertically  above. 

inferred  from  the  fact  that  when  the  leaflets  do  not  radiate 
from  the  summit  of  a  common  petiole,  or,  again,  when  there  is 
plenty  of  room,  from  the  sub-petioles  not  being  very  short,  the 
leaflets  sink  down  without  becoming  folded.  This  occurs  with 
the  leaflets  of  0.  sensitive,  Plumierii,  and  bupleurifolia. 

There  is  no  use  in  giving  a  long  list  of  the  many  species 
which  sleep  in  the  above  described  manner.  This  holds  good 
with  species  having  rather  fleshy  leaves,  like  those  of  0.  carnosa, 
or  large  leaves  like  those  of  0.  Ortcgef.il,  or  four  leaflets  like 
those  of  0.  variabilis.  There  are,  however,  some  species  which 
show  no  signs  of  sleep,  viz.,  0.  pentaphylla,  enneaphytta,  hirta, 
and  rubella.  We  will  now  describe  the  nature  of  the  movements 
in  some  of  the  species. 

Oxalis  acetose.Ua. — The  movement  of  a  leaflet,  together  with 
that  of  the  main  petiole,  are  shown  in  the  following  dia- 
gram (Fig.  128),  traced  between  11  A.M.  on  October  4th  and 
7.45  A.M.  on  the  5th.  After  5.30  P.M.  on  the  4th  the  leaflet  sank 
rapidly,  and  at  7  P.M.  depended  vertically.  For  some  time 
before  it  assumed  this  latter  position,  its  movements  could,  of 
course,  no  longer  be  traced  on  the  vertical  glass,  and  the 
broken  line  in  the  diagram  ought  to  be  extended  much  further 


326  MODIFIED   CIRCUMNUTATION. 

Fig.  128. 


CHAP.  VII. 


Oxalis   acetosella :  circumnutation 
nyc 


down  in  this  and  all  other 
cases.  By  6.45  A.M.  on  the 
following  morning  it  had 
risen  considerably,  and  con- 
tinued to  rise  for  the  next 
hour;  but,  judging  from 
other  observations,  it  would 
soon  have  begun  to  fall  again. 
Between  11  A.M.  and  5.30  P.M. 
the  leaflet  moved  at  least  four 
times  up  and  four  times 
down  before  the  great  noc- 
turnal fall  commenced;  it 
reached  its  highest  point  at 
noon.  Similar  observations 
were  made  on  two  other 
leaflets,  with  nearly  the  same 
results.  Sachs  and  Pfeifer 
have  also  described  briefly  * 
the  autonomous  movements 
of  the  leaves  of  this  plant. 

On  another  occasion  the 
petiole  of  a  leaf  was  secured 
to  a  little  stick  close  beneath 
the  leaflets,  and  a  filament 
tipped  with  a  bead  of  sealing- 
wax  was  affixed  to  the  mid- 
rib of  one  of  them,  and  a 
mark  was  placed  close  behind. 
At  7  P.M.,  when  the  leaflets 
were  asleep,  the  filament  de- 
pended vertically  down,  and 
the  movements  of  the  bead 
were  then  traced  till  10.40 
P.M.,  as  shown  in  the  fol- 
lowing diagram  (Fig.  129). 
We  here  see  that  the  leaflet 
moved  a  little  from  side  to 


tropic    movements  of    a   nearly 
full-grown    leaf,    with    filament    at- 
tached to  the  midrib  of  one  of  the   si(Je    as  wen   as   a   little  up 
leaflets;  traced  on  vertical  glass  dur-        d  d  ^^ 

ing  20  h.  4o  m. 

*  Sachs  in  'Flora,'  1863,  p.  470,  &c.;  Pfeffer,  'Die  Period.  Bewe- 
gungen,'  &c.,  1875,  p.  53. 


CHAP.  VII.  SLEEP   OF   LEAVES.  327 

Oxalis    Valdiviana.—The  leaves  resemble  those  of  the  last 
species,  and  the  movements  of  two  leaflets  (the  main  petioles  of 
both   having  been   secured)   were 
traced  during  two  days;    but  the  Fig-  129. 

tracings  are  not  given,  as  they 
resembled  that  of  0.  acetosella,  with 
the  exception  that  the  up  and 
down  oscillations  were  not  so  fre- 
quent during  the  day,  and  there  Oxalis  acetosella:  circumnuta- 

was  more  lateral  movement,  so  that       *ion  °/  leaflet  whfen  .asIe«P  ? 
...  1-1  traced     on      vertical     glass 

broader    ellipses    were    described.       during  3  h.  40  m. 
The  leaves  awoke  early  in  the  morn- 
ing, for  by  6.45  A.M.  on  June  12th  and  13th  they  had  not  only 
risen  to  their  full  height,  but  had  already  begun  to  fall,  that  is, 
they  were  circumnutating.     We  have  seen  in  the  last  chapter 
that  the  cotyledons,  instead  of  sinking,  rise   up  vertically  at 
night. 

Oxalis  Orteyesii. — The  large  leaves  of  this  plant  sleep  like 
those  of  the  previous  species.  The  main  petioles  are  long,  and 
that  of  a  young  leaf  rose  20°  between  noon  and  10  P.M.,  whilst 
the  petiole  of  an  older  leaf  rose  only  13°.  Owing  to  this  rising 
of  the  petioles,  and  the  vertical  sinking  of  the  large  leaflets, 
the  leaves  become  crowded  together  at  night,  and  the  whole 
plant  then  exposes  a  much  smaller  surface  to  radiation  than 
during  the  day. 

Oxalis  Plumierii. — In  this  species  the  three  leaflets  do  not 
surround  the  summit  of  the  petiole,  but  the  terminal  leaflet 
projects  in  the  line  of  the  petiole,  with  a  lateral  leaflet  on  each 
side.  They  all  sleep  by  bending  vertically  downwards,  but 
do  not  become  at  all  folded.  The  petiole  is  rather  long,  and, 
one  having  been  secured  to  a  stick,  the  movement  of  the  terminal 
leaflet  was  traced  during  45  h.  on  a  vertical  glass.  It  moved 
in  a  very  simple  manner,  sinking  rapidly  after  5  P.M.,  and 
rising  rapidly  early  next  morning.  During  the  middle  of  the  day 
it  moved  slowly  and  a  little  laterally.  Consequently  the  ascend- 
ing and  descending  lines  did  not  coincide,  and  a  single  great 
ellipse  was  formed  each  day.  There  was  no  other  evidence  of 
circumnutation,  and  this  fact  is  of  interest,  as  we  shall  here- 
after see. 

Oxalis  sensitiva. — The  leaflets,  as  in  the  last  species,  bend 
vertically  down  at  night,  without  becoming  folded.     The  much 
elongated  main  petiole  rises  considerably  in  the  evening,  but  in 
15 


328 


MODIFIED  CIKCUMNUTATION.          CHAP.  VII. 


FiS-  13°- 


some  very  young  plants  the  rise  did  not  commence  until  late 

at  night.    We  have  seen  that  the  cotyledons,  instead  of  sink- 

ing like  the  leaflets,  rise  up  vertically  at  night. 

Oxalis  buphurifolia.  —  This  species 
is  rendered  remarkable  by  the  petioles 
being  foliaceous,  like  the  phyllodes 
of  many  Acacias.  The  leaflets  are 
bmall,  of  a  paler  green  and  more 
tender  consistence  than  the  folia- 
ceous  petioles.  The  leaflet  which  was 
observed  was  •  55  inch  in  length,  and 
was  borne  by  a  petiole  2  inches  long 
and  '3  inches  broad.  It  may  be 
suspected  that  the  leaflets  are  on  the 
road  to  abortion  or  obliteration,  as 
has  actually  occurred  with  those  of 
another  Brazilian  species,  0.  rusci- 
formis.  Nevertheless,  in  the  present 
species  the  nyctitropic  movements 
are  perfectly  performed.  The  folia- 
ceous  petiole  was  first  observed 
during  48  h.,  and  found  to  be  in 
continued  circumnutation,  as  shown 
in  the  accompanying  figure  (Fig. 
130).  It  rose  during  the  day  and 
early  part  of  the  night,  and  fell 

Oxalis  bupleurifolia  :  circum-    during  the  remainder  of  the  night 

StfiifiST  oPb8:  -d  <-*  ™™  b<"  the  move- 

liquely  across  end  of  petiole;  ment  was  not  sufficient  to  be  called 

movements  traced  on  ver-  sleep.    The  ascending  and  descend- 

tical  glass  from  9.  A.M.  June  ing  lines  did  not  coinci(ie  so  that  an 

Join  to    o.oU    A.M.     zotn.  ,,.                  _           -ill           mi 

Apex  of  leaflet  4*    inches  elllPse  was  formed  each  day.    There 

from  the  glass,  so  movement  was    but    little    zigzagging  ;    if    the 

not  much  magnified.   Plant  filament  had  been  fixed  longitudi- 


f 


that  there  was  more  lateral  move- 
ment than  appears  in  the  diagram. 

A  terminal  leaflet  on  another  leaf  was  next  observed  (the 
petiole  being  secured),  and  its  movements  are  shown  in 
Fig.  131.  During  the  day  the  leaflets  are  extended  horizon- 
tally, and  at  night  depend  vertically  ;  and  as  the  petiole  rises 
daring  the  day  the  leaflets  have  to  bend  down  in  the  evening 


ClIAP.   VII. 


SLEEP    OF   LEAVES. 


329 


more  than  90°,  so  as  to  assume  at  night  their  vertical  position. 
On  the  first  day  the  leaflet  simply  moved  up  and  down ;  on  the 


Fig.  131. 


second  day  it  plainly  circumnutated  between  8  A.M.  and  4.30  P.M., 
after  which  hour  the  great  evening  fall  commenced. 


330 


MODIFIED   CIKCUMNUTATION.          CHAP.  VII. 


Averrhoa  UlimU  (Oxalidse).— It  has  long  been  known,*  firstly, 
that  the  leaflets  in  this  genus  sleep ;  secondly,  that  they  move 
spontaneously  during  the  day ;  and  thirdly,  that  they  are  sensi- 
tive to  a  touch ;  but  in  none  of  these  respects  do  they  differ 
essentially  from  the  species  of  Oxalis.  They  differ,  however,  as 
Mr.  E.  I.  Lynch  f  has  lately  shown,  in  their  spontaneous  move- 
ments being  strongly  marked.  In  the  case  of  A.  bilimbi,  it  is  a 
wonderful  spectacle  to  behold  on  a  warm  sunny  day  the  leaflets 
one  after  the  other  sinking  rapidly  downwards,  and  again 
ascending  slowly.  Their  movements  rival  those  of  Desmodium 
gyrans.  At  night  the  leaflets  hang  vertically  down;  and  now 

Fig.  132. 


Aveirhox  bilimbi:  leaf  asleep  ;  drawing  reduced. 

they  are  motionless,  but  this  may  be  due  to  the  opposite  ones 
being  pressed  together  (Fig.  132).  The  main  petiole  is  in  con- 
stant movement  during  the  day,  but  no  careful  observations  were 
made  on  it.  The  following  diagrams  are  graphic  representa- 
tions of  the  variations  in  the  angle,  which  a  given  leaflet  makes 
with  the  vertical.  The  observations  were  made  as  follows. 
The  plant  growing  in  a  pot  was  kept  in  a  high  temperature, 
the  petiole  of  the  leaf  to  be  observed  pointing  straight  at 
the  observer,  being  separated  from  him  by  a  vertical  pane  of 
glass.  The  petiole  was  secured  so  that  the  basal  joint,  or  pul- 
vinus,  of  one  of  the  lateral  leaflets  was  at  the  centre  of  a  gradu- 
ated arc  placed  close  behind  the  leaflet.  A  fine  glass  filament 
was  fixed  to  the  leaf,  so  as  to  project  like  a  continuation  of  the 


*  Dr.  Bruce,  « Philosophical  Trans.,'  1785,  p.  356. 
4  Journal  Linn.  Soc.,'  vol.  xvi.  1877,  p.  231. 


CHAP.  VII. 


SLEEP  OF   LEAVES. 


331 


midrib.    This  filament  acted  as  an  index;  and  as  the  leaf  rose 
and  fell,  rotating  about  its  basal  joint,  its  angular  movement 

Fig.  133. 


Avcrrhoa  bilimbi :  angular  movements  of  a  leaflet  during  its  evening 
descent,  when  going  to  sleep.     Temp.  78°-81°  F. 

could  be  recorded  by  reading  off  at  short  interval*  of  time  the 
position  of  the  glass  filament  on  the  graduated  arc.     In  order 


332  MODIFIED  CIKCUMNUTATION.         CHAP.  VII. 

to  avoid  errors  of  parallax,  all  readings  were  made  by  looking 
through  a  small  ring  painted  on  the  vertical  glass,  in  a  line 
with  the  joint  of  the  leaflet  and  the  centre  of  the  graduated  arc. 
In  the  following  diagrams  the  ordinates  represent  the  angles 
which  the  leaflet  made  with  the  vertical  at  successive  instants.* 
It  follows  that  a  fall  in  the  curve  represents  an  actual  dropping 
of  the  leaf,  and  that  the  zero  line  represents  a  vertically  de- 
pendent position.  Fig.  133  represents  the  nature  of  the  move- 
ments which  occur  in  the  evening,  as  soon  as  the  leaflets  begin 
to  assume  their  nocturnal  position.  At  4.55  P  M.  the  leaflet 
formed  an  angle  of  85°  with  the  vertical,  or  was  only  5°  below 
the  horizontal ;  but  in  order  that  the  diagram  might  get  into 
our  page,  the  leaflet  is  represented  falling  from  75°  instead 
of  85°.  Shortly  after  6  P.M.  it  hung  vertically  down,  and  had 
attained  its  nocturnal  position.  Between  6.10  and  6.35  P.M.  it 
performed  a  number  of  minute  oscillations  of  about  2°  each, 
occupying  periods  of  4  or  5  m.  The  complete  state  of  rest  of 
the  leaflet  which  ultimately  followed  is  not  shown  in  the  dia- 
gram. It  is  manifest  that  each  oscillation  consists  of  a  gradual 
rise,  followed  by  a  sudden  fall.  Each  time  the  leaflet  fell,  it 
approached  nearer  to  the  nocturnal  position  than  it  did  on  the 
previous  fall.  The  amplitude  of  the  oscillations  diminished, 
while  the  periods  of  oscillation  became  shorter. 

In  bright  sunshine  the  leaflets  assume  a  highly  inclined  de- 
pendent position.  A  leaflet  in  diffused  light  was  observed  rising 
for  25  m.  A  blind  was  then  pulled  up  so  that  the  plant  was 
brightly  illuminated  (BE  in  Fig.  134),  and  within  a  minute  it 
began  to  fall,  and  ultimately  fell  47°,  as  shown  in  the  diagram. 
This  descent  was  performed  by  six  descending  steps,  precisely 
f-imilar  to  those  by  which  the  nocturnal  fall  is  effected.  The 
plant  was  then  again  shaded  (SH),  and  a  long  slow  rise  occurred 
until  another  series  of  falls  commenced  at  BR',  when  the  sun 
was  again  admitted.  In  this  experiment  cool  air  was  allowed 
to  enter  by  the  windows  being  opened  at  the  same  time  that 
the  blinds  were  pulled  up,  so  that  in  spite  of  the  sun  shining 
on  the  plant  the  temperature  was  not  raised. 

The  effect  of  an  increase  of  temperature  in  diffused  light  is 

*  In  all  the  diagrams  1  mm.  in  ment.     In  Figs.  133  and  134  the 

the  horizontal  direction  represents  temperature  is  represented  (along 

one  minute  of  time.     Each  mm.  the  ordinates)  in  the  scale  of  1 

in   the   vertical   direction   repre-  mm.  to   each   O'l^C.      In   Fig. 

scnts  one  degree  of  angular  move-  135  each  mm.  equals  0*2°  F. 


CHAP.  VII.  SLEEP   OF   LEAVES.  333 

shown  in  Fig.  135.  The  temperature  began  to  rise  at  11.35 
A.M.  (in  consequence  of  the -fire  being  lighted),  but  by  12.42  a 
marked  fall  had  occurred.  It  may  be  seen  in  the  diagram  that 
when  the  temperature  was  highest  there  were  rapid  oscillations 

Fig.  134. 


Arerrhoa  liUmbi:  angular  movements  of  leaflet  during  a  change  from 
bright  illumination  to  shade  ;  temperature  (broken  line)  remaining 
nearly  the  same. 

of  small  amplitude,  the  mean  position  of  the  leaflet  being  at  the 
time  nearer  the  vertical.  When  the  temperature  began  to  fall, 
the  oscillations  became  slower  and  larger,  and  the  mean  position 
of  the  leaf  again  approached  the  horizontal.  The  rate  of  oscil- 
lation was  sometimes  quicker  than  is  represented  in  the  above 
diagram.  Thus,  when  the  temperature  was  between  31°  end 


334:  MODIFIED   CIRCUMNUTATION. 

Fig.  135. 


CHAP.  VII- 


CHAP.  VII. 


SLEEP   OF   LEAVES. 


335 


136. 


32°  C.,  14  oscillations  of  a  few  degrees  occurred  in  19m.  On 
the  other  hand,  an  oscillation  may  be  much  slower ;  thus  a  leaflet 
was  observed  (temperature  25°  0.)  to 
rise  during  40  in.  before  it  fell  and 
completed  its  oscillation. 

PorUeria  liygrometrica  (Zygophyllese) 
— The  leaves  of  this  plant  (Chilian 
form)  are  from  1  to  l£  inches  in  length, 
and  bear  as  many  as  16  or  17  small 
leaflets  on  each  side,  which  do  not 
stand  opposite  one  another.  They  are 
articulated  to  the  petiole,  and  the 
petiole  to  the  branch  by  a  pulvinus. 
We  must  premise  that  apparently  two 
forms  are  confounded  under  the  same 
name  :  the  leaves  on  a  bush  from  Chili, 
which  was  sent  to  us  from  Kew,  bore 
many  leaflets,  whilst  those  on  plants 
in  the  Botanic  Garden  at  Wiirzburg 
bore  only  8  or  9  pairs ;  and  the  whole 
character  of  the  bushes  appeared  some- 
what different.  We  shall  also  see  that 
they  differ  in  a  remarkable  physio- 
logical peculiarity.  On  the  Chilian 
plant  the  petioles  of-  the  younger  leaves 
on  upright  branches,  stood  horizontally 
during  the  day,  and  at  night  sank 
down  vertically  so  as  to  depend  parallel 
and  close  to  the  branch  beneath.  The 
petioles  of  rather  older  leaves  did  not 
become  at  night  vertically  depressed, 
but  only  -  highly  inclined.  In  one 
instance  we  found  a  branch  which  had 
grown  perpendicularly  downwards, 
and  the  petioles  on  it  moved  in  the  same 
direction  relatively  to  the  branch  as 
just  stated,  and  therefore  moved  up- 
wards. On  horizontal  branches  the 
younger  petioles  likewise  move  at  night 
in  the  same  direction  as  before,  that  is, 
towards  the  branch,  and  are  consequently  then  extended  hori- 
zontally; but  it  is  remarkable  that  the  older  petioles  on  the 


Policrla  hygrometrica :  cir- 
cumnutation  and  nycti- 
tropic  movements  of  pe- 
tiole of  leaf,  traced  from 
9.35  A.M.  July  7th  to 
about  midnight  on  the 
8th.  Apex  of  leaf  7£ 
inches  from  the  vertical 
glass.  Temp.  19|°-20£°C. 


336  MODIFIED   CIBCUMXUTATION.  CHAP.  VII. 

same  branch,  though  moving  a  little  in  the  same  direction,  also 
bend  downwards;  they  thus  occupy  a  somewhat  different  posi- 
tion, relatively  to  the  centre  of  the  earth  and  to  the  branch,  from 
that  of  the  petioles  on  the  upright  branches.  With  respect  to 
the  leaflets,  they  move  at  night  towards  the  apex  of  the  petiole 
until  their  midribs  stand  nearly  parallel  to  it ;  and  they  then 
lie  neatly  imbricated  one  over  the  other.  Thus  half  of  the  upper 
surface  of  each  leaflet  is  in  close  contact  with  half  of  the  lower 
surface  of  the  one  next  in  advance ;  and  all  the  leaflets,  except- 
ing the  basal  ones,  have  the  whole  of  their  upper  surfaces  and 
half  of  their  lower  surfaces  well  protected.  Those  on  the  oppo- 
site sides  of  the  same  petiole  do  not  come  into  close  contact 
at  night,  as  occurs  with  the  leaflets  of  so  many  LeguminosaB, 
but  are  separated  by  an  open  furrow ;  nor  could  they  exactly 
coincide,  as  they  stand  alternately  with  respect  to  one  another. 

The  circumnutation  of  the  petiole  of  a  leaf  f  of  an  inch  in 
length,  on  an  upright  branch,  was  observed  during  36  h., 
and  is  shown  in  the  preceding  diagram  (Fig.  136).  On  the 
first  morning,  the  leaf  fell  a  little  and  then  rose  until  1  P.M  , 
and  this  was  probably  due  to  its  being  now  illuminated  through 
a  skylight  from  above ;  it  then  circumnutated  on  a  very  small 
scale  round  the  same  spot  until  about  4  P.M.,  when  the  great 
evening  fall  commenced.  During  the  latter  part  of  the  night  or 
very  early  on  the  next  morning  the  leaf  rose  again.  On  the 
second  day  it  fell  during  the  morning  till  1  P.M.,  and  this  no 
doubt  is  its  normal  habit.  From  1  to  4  P.M.  it  rose  in  a  zigzag 
line,  and  soon  afterwards  the  great  evening  fall  commenced.  It 
thus  completed  a  double  oscillation  during  the  24  h. 

The  specific  name  given  to  this  plant  by  Kuiz  and  Pa  von,  indi- 
cates that  in  its  native  arid  home  it  is  affected  in  some  manner 
by  the  dryness  or  dampness  of  the  atmosphere.*  In  the  Botanic 
Garden  at  Wiirzburg,  there  was  a  plant  in  a  pot  out  of  doors 
which  was  daily  watered,  and  another  in  the  open  ground  which 
was  never  watered.  After  some  hot  and  dry  weather  there  was 
a  great  difference  in  the  state  of  the  leaflets  on  these  two  plants ; 
those  on  the  unwatered  plant  in  the  open  ground  remaining  half, 


*  '  Systema  Veg.  Florae  Peru-  about    its    power    of    foretelling 

vianse  et  Chilensis,'  torn.  i.  p.  95,  changes  in  the  weather ;   and  it 

1798.     We  cannot  understand  the  appears  as  if  the  brightness  of  the 

account  given  by  the  authors  of  sky  largely  determined  tho  open- 

the  behaviour  of  this  plant  in  its  ing  and  closing  of  the  leaflets, 
native    home.      There    is    much 


CHAP.  VII.  SLEEP  OF  LEAVES.  337 

or  even  quite,  closed  during  the  day.  But  twigs  cut  from  this 
bush,  with  their  ends  standing  in  water,  or  wholly  immersed  in 
it,  or  kept  in  damp  air  under  a  bell-glass,  opened  their  leaves 
though  exposed  to  a  blazing  sun;  whilst  those  on  the  plant 
in  the  ground  remained  closed.  The  leaves  on  this  same  plant, 
after  some  heavy  rain,  remained  open  for  two  days;  they  then 
became  half  closed  during  two  days,  and  after  an  additional 
day  were  quite  closed.  This  plant  was  now  copiously  watered, 
and  on  the  following  morning  the  leaflets  were  fully  ex- 
panded. The  other  plant  growing  in  a  pot,  after  having  been 
exposed  to  heavy  rain,  was  placed  before  a  window  in  the  Labo- 
ratory, with  its  leaflets  open,  and  they  remained  so  during  the 
daytime  for  48  h. ;  but  after  an  additional  day  were  half  closed. 
The  plant  was  then  watered,  and  the  leaflets  on  the  two  following 
days  remained  open.  On  the  third  day  they  were  again  half 
closed,  but  on  being  again  watered  remained  open  during  the 
two  next  days.  From  these  several  facts  we  may  conclude  that 
the  plant  soon  feels  the  want  of  water ;  and  that  as  soon  as  this 
occurs,  it  partially  or  quite  closes  its  leaflets,  which  in  their 
then  imbricated  condition  expose  a  small  surface  to  evaporation. 
It  is  therefore  probable  that  this  sleep-like  movement,  which 
occurs  only  when  the  ground  is  dry,  is  an  adaptation  against 
the  loss  of  moisture. 

A  bush  about  4  feet  in  height,  a  native  of  Chili,  which  was 
thickly  covered  with  leaves,  behaved  very  differently,  for  during 
the  day  it  never  closed  its  leaflets.  On  July  6th  the  earth  in 
the  small  pot  in  which  it  grew  appeared  extremely  dry,  and 
it  was  given  a  very  little  water.  After  21  and  22  days  (on 
the  27th  and  28th),  during  the  whole  of  which  time  the  plant 
did  not  receive  a  drop  of  water,  the  leaves  began  to  droop,  but 
they  showed  no  signs  of  closing  during  the  day.  It  appeared 
almost  incredible  that  any  plant,  except  a  fleshy  one,  could 
have  kept  alive  in  soil  so  dry,  which  resembled  the  dust  on 
a  road.  On  the  29th,  when  the  bush  was  shaken,  some  leaves 
fell  off,  and  the  remaining  ones  were  unable  to  sleep  at  night. 
It  was  therefore  moderately  watered,  as  well  as  syringed,  late  in 
the  evening.  On  the  next  morning  (30th)  the  bush  looked  as  fresh 
as  ever,  and  at  night  the  leaves  went  to  sleep.  It  may  be  added 
that  a  small  branch  while  growing  on  the  bush  was  enclosed, 
by  means  of  a  curtain  of  bladder,  during  13  days  in  a  large 
bottle  half  full  of  quicklime,  so  that  the  air  within  must  have  been 
intensely  dry ;  yet  the  leaves  on  this  branch  did  not  suffer  in  the 


338  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

least,  and  did  not  close  at  all  during  the  hottest  days.  Another 
trial  was  made  with  the  same  bush  on  August  2nd  and  6th  (the  soil 
appearing  at  this  latter  date  extremely  dry),  for  it  was  exposed 
out  of  doors  during  the  whole  day  to  the  wind,  but  the  leaflets 
showed  no  signs  of  closing.  The  Chilian  form  therefore  differs 
widely  from  the  one  at  Wiirzburg,  in  not  closing  its  leaflets 
when  suffering  from  the  want  of  water;  and  it  can  live  for  a 
surprisingly  long  time  without  water. 

Tropceolum  m.-ijus  (?)  (cultivated  var.)  (Tropseolese). — Several 
plants  in  pots  stood  in  the  greenhouse,  and  the  blades  of 
the  leaves  which  faced  the  front-lights  were  during  the  day 
highly  inclined  and  at  night  vertical;  whilst  the  leaves  on 
the  back  of  the  pots,  though  of  course  illuminated  through 
the  roof,  did  not  become  vertical  at  night.  \Ve  thought,  at  first, 
that  this  d  iference  in  their  positions  was  in  some  manner 
due  to  heliotropism,  for  the  leaves  are  highly  heliotropic.  The 
true  explanation,  however,  is  that  unless  they  are  well  illu- 
minated during  at  least  a  part  of  the  day  they  do  not  sleep  at 
night;  and  a  little  difference  in  the  degree  of  illumination  deter- 
mines whether  or  not  they  shall  become  vertical  at  night.  We 
have  observed  no  other  so  well-marked  a  case  as  this,  of  the 
influence  of  previous  illumination  on  nyctitropic  movements. 
The  leaves  present  also  another  peculiarity  in  their  habit  of 
rising  or  awaking  in  the  morning,  being  more  strongly  fixed  or 
inherited  than  that  of  sinking  or  sleeping  at  night.  The  move- 
ments are  caused  by  the  bending  of  an  upper  part  of  the  petiole, 
between  \  and  1  inch  in  length ;  but  the  part  close  to  the  blade, 
for  about  £  of  an  inch  in  length,  does  not  bend  and  always 
remains  at  right  angles  to  the  blade.  The  bending  portion  does 
not  present  any  external  or  internal  difference  in  structure 
from  the  rest  of  the  petiole.  We  will  now  give  the  experiments 
on  which  the  above  conclusions  are  founded. 

A  large  pot  with  several  plants  was  brought  on  the  morning 
of  Sept.  3rd  out  of  the  greenhouse  and  placed  before  a  north-east 
window,  in  the  same  position  as  before  with  respect  to  the  light, 
as  far  as  that  was  possible.  On  the  front  of  the  plants,  24  leaves 
were  marked  with  thread,  some  of  which  had  their  blades  hori- 
zontal, but  the  greater  number  were  inclined  at  about  45°, 
beneath  the  horizon;  at  night  all  these,  without  exception, 
became  vertical.  Early  on  the  following  morning  (4th)  they 
reassumed  their  former  positions,  and  at  night  again  became 
vertical.  On  the  5th  the  shutters  were  opened  at  6.15  A.M.,  and 


CHAP.  VII.  SLEEP   OF  LEAVES.  339 

by  8.18  A.M.,  after  the  leaves  had  been  illuminated  for  2  h.  3  m., 
and  had  acquired  their  diurnal  position,  they  were  placed  in  a 
dark  cupboard.  They  were  looked  at  twice  during  the  day  and 
thrice  in  the  evening,  the  last  time  at  10  30  P.M.,  and  not  one  had 
become  vertical.  At  8  A.M.  on  the  following  morning  (6th)  they 
still  retained  the  same  diurnal  position,  and  were  now  replaced 
before  the  north-east  window.  At  night  all  the  leaves  which 
had  faced  the  light  had  their  petioles  curved  and  their  blades 
vertical ;  whereas  none  of  the  leaves  on  the  back  of  the  plants, 
although  they  had  been  moderately  illuminated  by  the  diffused 
light  of  the  room,  were  vertical.  They  were  now  at  night  placed 
in  the  same  dark  cupboard ;  at  9  A.M.  on  the  next  morning  (7th) 
all  those  which  had  been  asleep  had  reassumed  their  diurnal 
position.  The  pot  was  then  placed  for  3  h.  in  the  sunshine,  so 
as  to  stimulate  the  plants;  at  noon  they  were  placed  before  the 
same  north-east  window,  and  at  night  the  leaves  slept  in  the 
usual  manner  and  awoke  on  the  following  morning.  At  noon  on 
this  day  (8th)  the  plants,  after  having  been  left  before  the  north- 
east window  for  5  h.  45  m.  and  thus  illuminated  (though  not 
brightly,  as  the  sky  was  cloudy  during  the  whole  time),  were 
replaced  in  the  dark  cupboard,  and  at  3  p.  M.  the  position  of  the 
leaves  was  very  little,  if  at  all,  altered,  so  that  they  are  not 
quickly  affected  by  darkness ;  but  by  10.15  P.M.  all  the  leaves 
which  had  faced  the  north-east  sky  during  the  5h.  45m.  of 
illumination  stood  vertical,  whereas  those  on  the  back  of  the 
plant  retained  their  diurnal  position.  On  the  following  morning 
(9th)  the  leaves  awoke  as  on  the  two  former  occasions  in  the  dark, 
and  they  were  kept  in  the  dark  during  the  whole  day ;  at  night 
a  very  few  of  them  became  vertical,  and  this  was  the  one  in- 
stance in  which  we  observed  any  inherited  tendency  or  habit  in 
this  plant  to  sleep  at  the  proper  time.  That  it  was  real  sleep 
was  shown  by  these  same  leaves  reassuming  their  diurnal  posi- 
tion on  the  following  morning  (10th)  >  whilst  still  kept  in  the 
dark. 

The  pot  was  then  (9.45  A.M.  10th)  replaced,  after  having  been 
kept  for  36  h.  in  darkness,  before  the  north-east  window ;  and  at 
night  the  blades  of  all  the  leaves  (excepting  a  few  on  the  back  of 
the  plants)  became  conspicuously  vertical. 

At  6.45  A.M.  (llth)  after  the  plants  had  been  illuminated  on  the 
same  side  as  before  during  only  25  m.,  the  pot  was  turned  round, 
so  that  the  leaves  which  had  faced  the  light  now  faced  the 
interior  of  the  room,  and  not  one  of  these  went  to  sleep  at  night; 


340  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

whilst  some,  but  not  many,  of  those  which  had  formerly  stood 
facing  the  back  of  the  room  and  which  had  never  before  been 
well  illuminated  or  gone  to  sleep,  now  assumed  a  vertical  posi- 
tion at  night.  On  the  next  day  (12th)  the  plant  was  turned 
round  into  its  original  position,  so  that  the  sa^e  leaves  faced 
the  light  as  formerly,  and  these  now  went  to  sleep  in  the  usual 
manner.  We  will  only  add  that  with  some  young  seedlings 
kept  in  the  greenhouse,  the  blades  of  the  first  pair  of  true  leaves 
(the  cotyledons  being  hypogean)  stood  during  the  day  almost 
horizontally  and  at  night  almost  vertically. 

A  few  observations  were  subsequently  made  on  the  circum- 
nutation  of  three  leaves,  whilst  facing  a  north-east  window ;  but 
the  tracings  are  not  given,  as  the  leaves  moved  somewhat 
towards  the  light.  It  was,  however,  manifest  that  they  rose 
and  fell  more  than  once  during  the  daytime,  the  ascending  and 
descending  lines  being  in  parts  extremely  zigzag.  The  nocturnal 
fall  commenced  about  7  P.M.,  and  the  leaves  had  risen  consider- 
ably by  6.45  A.M.  on  the  following  morning. 

Leguminosce. — This  Family  includes  many  more  genera  with 
sleeping  species  than  all  the  other  families  put  together.  The 
number  of  the  tribes  to  which  each  genus  belongs,  according  to 
Bentham  and  Hooker's  arrangement,  has  been  added. 

Crotolaria  (sp.  ?)  (Tribe  2). — This  plant  is  monophyllous,  and 
we  are  informed  by  Mr.  T.  Thiselton  Dyer  that  the  leaves  rise 
up  vertically  at  night  and  press  against  the  stem. 

Lupinus  (Tribe  2). — The  palmate  or  digitate  leaves  of  the 
species  in  this  large  genus  sleep  in  three  different  manners. 
One  of  the  simplest,  is  that  all  the  leaflets  become  steeply  in- 
clined downwards  at  night,  having  been  during  the  day  ex- 
tended horizontally.  This  is  shown  in  the  accompanying 
figures  (Fig.  137),  of  a  leaf  of  L.  pilosus,  as  seen  during  the 
day  from  vertically  above,  and  of  another  leaf  asleep  with  the 
leaflets  inclined  downwards.  As  in  this  position  they  are 
crowded  together,  and  as  they  do  not  become  folded  like  those 
in  the  genus  Oxalis,  they  cannot  occupy  a  vertically  dependent 
position  ;  but  they  are  often  inclined  at  an  angle  of  50°  beneath 
the  horizon.  In  this  species,  whilst  the  leaflets  are  sinking, 
the  petioles  rise  up,  in  two  instances  when  the  angles  were 
measured  to  the  extent  of  23°.  The  leaflets  of  L.  sub-carnosus  and 
arboreus,  which  were  horizontal  during  the  day,  sank  down,  at 
night  in  nearly  the  same  manner ;  the  former  to  an  angle  of  38°, 
and  the  latter  of  36°,  beneath  the  horizon ;  but  their  petioles 


CHAP.  VII.  SLEEP  OF   LEAVES.  341 

did  not  move  in  any  plainly  perceptible  degree.  It  is,  however, 
quite  possible,  as  we  shall  presently  see,  that  if  a  large  number 
of  plants  of  the  three  foregoing  and  of  the  following  species 

Fig.  137. 


A. 

Lupinus  pilosus  :  A,  leaf  seen  from  vertically  above  in  daytime;  B,  leaf 
asleep,  seen  laterally  at  night. 

were  to  be  observed  at  all  seasons,  some  of  the  leaves  would  be 
found  to  sleep  in  a  different  manner. 

In  the  two  following  species  the  leaflets,  instead  of  moving 
downwards,  rise  at  night.  With  L.  Hartwegii  some  stood  at 
noon  at  a  mean  angle  of  36°  above  the  horizon,  and  at  night 
at  51°,  thus  forming  together  a  hollow  cone  with  moderately 
steep  sides.  The  petiole  of  one  leaf  rose  14°  and  of  a  second 
11°  at  night.  With  L.  luteus  a  leaflet  rose  from  47°  at  noon  to 
65°  above  the  horizon  at  night,  and  another  on  a  distinct  leaf 
rose  from  45°  to  69°.  The  petioles,  however,  sink  at  night  to 
a  small  extent,  viz.,  in  three  instances  by  2°,  6°,  and  9°  30'. 
Owing  to  this  movement  of  the  petioles,  the  outer  and  longer 
leaflets  have  to  bend  up  a  little  more  than  the  shorter  and  inner 
ones,  in  order  that  all  should  stand  symmetrically  at  night. 
We  shall  presently  see  that  some  leaves  on  the  same  individual 
plants  of  L.  luteus  sleep  in  a  very  different  manner. 

We  now  come  to  a  remarkable  position  of  the  leaves 
when  asleep,  which  is  common  to  several  species  of  Lupines. 
On  the  same  leaf  the  shorter  leaflets,  which  generally  face  the 
centre  of  the  plant,  sink  at  night,  whilst  the  longer  ones 
on  the  opposite  side  rise;  the  intermediate  and  lateral  ones 
merely  twisting  on  their  own  axes.  But  there  is  some  variability 
with  respect  to  which  leaflets  rise  or  fall.  As  might  have  been 
expected  from  such  diverse  and  complicated  movements,  the- 


342 


MODIFIED   C1RCUMNUTATION. 


CHAP.  VIT. 


base  of  each  leaflet  is  developed  (at  least  in  the  case  of  L.  luteus) 
into  a  pulvimis.  The  result  is  that  all  the  leaflets  on  the 
same  leaf  stand  at  night  more  or  less  highly  inclined,  or  even 
quite  vertically,  forming  in  this  latter  case  a  vertical  star.  This 
occurs  with  the  leaves  of  a  species  purchased  under  the  name  of 

Fig.  138 


Lupinus  pubescens:  A,  leaf  viewed  laterally  during  the  day;  B,  same  leaf 
at  night ;  C,  another  leaf  with  the  leaflet  forming  a  vertical  star  at 
night.  Figures  reduced. 

L.  pubescens ;  and  in  the  accompanying  figures  we  see  at  A  (Fig. 
138)  the  leaves  in  their  diurnal  position;  and  at  B  the  same 
plant  at  night  with  the  two  upper  leaves  having  their  leaflets 
almost  vertical.  At  C  another  leaf,  viewed  laterally,  is  shown 
with  the  leaflets  quite  vertical.  It  is  chiefly  or  exclusively  the 
youngest  leaves  which  form  at  night  vertical  stars.  But  there 


CHAP.  VII.  SLEEP   OF   LEAVES.  343 

is  much  variability  in  the  position  of  the  leaves  at  night  on  the 
same  plant ;  some  remaining  with  their  leaflets  almost  horizontal, 
others  forming  more  or  less  highly  inclined  or  vertical  stars,  and 
some  with  all  their  leaflets  sloping  downwards,  as  in  our  first 
class  of  cases.  It  is  also  a  remarkable  fact,  that  although  all  the 
plants  produced  from  the  same  lot  of  seeds  were  identical  in 
appearance,  yet  some  individuals  at  night  had  the  leaflets  of  all 
their  leaves  arranged  so  as  to  form  more  or  less  highly  inclined 
stars ;  others  had  th'em  all  sloping  downwards  and  never  forming 
a  star ;  and  others,  again,  retained  them  either  in  a  horizontal 
position  or  raised  them  a  little. 

We  have  as  yet  referred  only  to  the  different  positions  of  tne 
leaflets  of  L.  pubesceas  at  night ;  but  the  petioles  likewise  differ 
in  their  movements.  That  of  a  young  leaf  which  formed  a 
highly  inclined  star  at  night,  stood  at  noon  at  42°  above  the 
horizon,  and  during  the  night  at  72°,  so  had  risen  30°.  The 
petiole  of  another  leaf,  the  leaflets  of  which  occupied  a  similar 
position  at  night,  rose  only  6°.  On  the  other  hand,  the  petiole 
of  a  leaf  with  all  its  leaflets  sloping  down  at  night,  fell  at  this 
time  4°.  The  petioles  of  two  rather  older  leaves  were  subse- 
quently observed ;  both  of  which  stood  during  the  day  at  exactly 
the  same  angle,  viz.,  50°  above  the  horizon,  and  one  of  these  rose 
7°— 8°,  and  the  other  fell  3°— 4°  at  night, 

We  meet  with  cases  like  that  of  L.  pubescens  with  some  other 
species.  On  a  single  plant  of  L.  mutabilis  some  leaves,  which 
stood  horizontally  during  the  day,  formed  highly  inclined  stars 
at  night,  and  the  petiole  of  one  rose  7°.  Other  leaves  which 
likewise  stood  horizontally  during  the  day,  had  at  night  all  their 
leaflets  sloping  downwards  at  46°  beneath  the  horizon,  but 
their  petioles  had  hardly  moved.  Again,  L.  lut<  us  offered  a  still 
more  remarkable  case,  for  on  two  leaves,  the  leaflets  which  stood 
at  noon  at  about  45°  above  the  horizon,  rose  at  night  to  65°  and 
69°,  so  that  they  formed  a  hollow  cone  with  steep  sides.  Four 
leaves  on  the  same  plant,  which  had  their  leaflets  horizontal  at 
noon,  formed  vertical  stars  at  night ;  and  three  other  leaves 
equally  horizontal  at  noon,  had  all  their  leaflets  sloping  down- 
wards at  night.  So  that  the  leaves  on  this  one  plant  assumed 
at  night  three  different  positions.  Though  we  cannot  account 
for  this  fact,  we  can  see  that  such  a  stock  might  readily  give 
birth  to  species  having  widely  different  nyctitropic  habits. 

Little  more  need  be  said  about  the  sleep  of  the  species  of  Lu- 
pimis;  several,  namely,  L.  polypliyllus,  nanus,  Menziesii,  speciosus, 


344  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

and  albifrons,  though  observed  out  of  doors  and  in  the  green- 
house, did  not  change  the  position  of  their  leaves  sufficiently  at 
night  to  be  said  to  sleep.  From  observations  made  on  two 
sleeping  species,  it  appears  that,  as  with  Tropceolum  majus,  the 
leaves  must  be  well  illuminated  during  the  day  in  order  to  sleep 
at  night.  For  several  plants,  kept  all  day  in  a  sitting-room 
with  north-east  windows,  did  not  sleep  at  night ;  but  when  the 
pots  were  placed  on  the  following  day  out  of  doors,  and  were 
brought  in  at  night,  they  slept  in  the  usual  manner.  The  trial 
was  repeated  on  the  following  day  and  night  with  the  same 
result. 

Some  observations  were  made  on  the  circumnutation  of  the 
leaves  of  L.  luteus  and  arbureus.  It  will  suffice  to  say  that  the 
leaflets  of  the  latter  exhibited  a  double  oscillation  in  the  course 
of  '24  h. ;  for  they  fell  from  the  early  morning  until  10  15  A.M., 
then  rose  and  zigzagged  greatly  till  4  P.M.,  after  which  hour  the 
great  nocturnal  fall  commenced.  By  8  A.M.  on  the  following 
morning  the  leaflets  had  risen  to  their  proper  height.  We  have 
seen  in  the  fourth  chapter,  that  the  leaves  of  Lupinus  speciosus, 
which  do  not  sleep,  circumnutate  to  an  extraordinary  extent, 
making  many  ellipses  in  the  course  of  the  day. 

Cytisus  (Tribe  2),  Trigonella  and  Medicago  (Tribe  3).— Only 


A.  B. 

Medicago  marina  :  A,  leaves  during  the  day  ;  B,  leaves  asleep  at  night. 

a  few  observations  were  made  on  these  three  genera.  The 
petioles  on  a  young  plant,  about  a  foot  in  height,  of  Cytisus 
fragrans  rose  at  night,  on  one  occasion  23°  and  on  another  33°. 
The  three  leaflets  also  bend  upwards,  and  at  the  same  time 


CHAP.  VII.  SLEEP   OF   LEAVES.  345 

approach  each  other,  so  that  the  base  of  the  central  leaflet 
overlaps  the  bases  of  the  two  lateral  leaflets.  They  bend 
up  so  much  that  they  press  against  the  stem ;  and  on  looking 
down  on  one  of  these  )  oung  plants  from  vertically  above,  the 
lower  surfaces  of  the  leaflets  are  visible;  and  thus  their  upper 
surfaces,  in  accordance  with  the  general  rule,  are  best  protected 
from  radiation.  Whilst  the  leaves  on  these  young  plants  were 
thus  behaving,  those  on  an  old  bush  in  full  flower  did  not  sleep 
at  night. 

Tfigonella  Cretica  resembles  a  Melilotus  in  its  sleep,  which  will 
be  immediately  described.  According  to  M.  Eoyer,*  the  leaves 
of  Medicago  maculata  rise  up  at  night,  and  "  se  renversent  un 
peu  de  maniere  a  presenter  obliquement  au  ciel  leur  face  in- 
ferieure."  A  drawing  is  here  given  (Fig.  139)  of  the  leaves 
of  M.  marina  awake  and  asleep ;  and  this  would  almost  serve 
for  Cytisus  fragrans  in  the  same  two  states. 

Melilotus  (Tribe  3).— The  species  in  this  gemis  sleep  in  a 
remarkable  manner.  The  three  leaflets  of  each  leaf  twist  through 
an  angle  of  90°,  so  that  their  blades  stand  vertically  at  night 
with  one  lateral  edge  presented  to  the  zenith  (Fig.  140).  We 
shall  best  understand  the  other  and  more  complicated  move- 
ments, if  we  imagine  ourselves  always  to  hold  the  leaf  with  the 
tip  of  the  terminal  leaflet  pointed  to  the  north.  The  leaflets  in 
becoming  vertical  at  night  could  of  course  twist  so  that  their 
upper  surfaces  should  face  to  either  side ;  but  the  two  lateral 
leaflets  always  twist  so  that  this  surface  tends  to  face  the  north, 
but  as  they  move  at  the  same  time  towards  the  terminal  leaflet, 
the  upper  surface  of  the  one  faces  about  N.N.W.,  and  that  of 
the  other  N.N.E.  The  terminal  leaflet  behaves  differently,  for 
it  twists  to  either  side,  the  upper  surface  facing  sometimes  east 
and  sometimes  west,  but  rather  more  commonly  west  than  east. 
The  terminal  leaflet  also  moves  in  another  and  more  remarkable 
manner,  for  whilst  its  blade  is  twisting  and  becoming  vertical, 
the  whole  leaflet  bends  to  one  side,  and  invariably  to  the  side 
towards  which  the  upper  surface  is  directed;  so  that  if  this 
surface  faces  the  west  the  whole  leaflet  bends  to  the  west,  until 
it  comes  into  contact  with  the  upper  and  vertical  surface  of 
the  western  lateral  leaflet.  Thus  the  upper  surface  of  the 
terminal  and  of  one  of  the  two  lateral  leaflets  is  well  protected. 

The  fact  of  the  terminal  leaflet  twisting  indifferently  to  either 


;  Anuales  des  Sc.  Nat.  Bot.'  (5th  series),  ix.  1868,  p.  368. 


340 


MODIFIED   CIBCUMNUTATION. 


CHAP.  VII. 


side  and  afterwards  bending  to  the  same  side,  seemed  to  us  so 
remarkable,  that  we  endeavoured  to  discover  the  cause.  We 
imagined  that  at  the  commencement  of  the  movement  it  might 
be  determined  by  one  of  the  two  halves  of  the  leaflet  being 
a  little  heavier  than  the  other.  Therefore  bits  of  wood  were 
gummed  on  one  side  of  several  leaflets,  but  this  produced  no 
effect;  and  they  continued  to  twist  in  the  same  direction  as 

Fig.  140. 


Melilotus  officincilis :  A,  leaf  during  the  daytime.  B,  another  leaf  asleep. 
C,  a  leaf  asleep  as  viewed  from  vertically  above  ;  but  in  this  case  the 
terminal  leaflet  did  not  happen  to  be  in  such  close  contact  with  the 
lateral  one,  as  is  usual. 

they  hal  previously  done.  In  order  to  discover  whether  the 
same  leaflet  twisted  permanently  in  the  same  direction,  black 
threads  were  tied  to  20  leaves,  the  terminal  leaflets  of  which 
twisted  so  that  their  upper  surfaces  faced  west,  and  14  white 
threads  to  leaflets  which  twisted  to  the  east.  These  were  ob- 
served occasionally  during  14  days,  and  they  all  continued,  with 
a  single  exception,  to  twist  and  bend  in  the  same  direction ;  for 


CHAP.  VII.  SLEEP  OF   LEAVES.  347 

one  leaflet,  which  had  originally  faced  east,  was  observed  after 
9  days  to  face  west.  The  seat  of  both  the  twisting  and  bending 
movement  is  in  the  pulvinns  of  the  sub-petioles. 

We  believe  that  the  leaflets,  especially  the  two  lateral  ones, 
in  performing  the  above  described  complicated  movements 
generally  bend  a  little  downwards ;  but  we  are  not  sure  of  this, 
for,  as  far  as  the  main  petiole  is  concerned,  its  nocturnal  move- 
ment is  largely  determined  by  the  position  which  the  leaf 
happens  to  occupy  during  the  day.  Thus  one  main  petiole  was 
observed  to  rise  at  night  59°,  whilst  three  others  rose  only  7° 
and  9°.  The  petioles  and  sub-petioles  are  continually  circum- 
nutating  during  the  whole  21  h.,  as  we  shall  presently  see. 

The  leaves  of  the  following  15  species,  M.  ojficinalis^uaveolens, 
parviflora,  alba,  wftsta,  dtntat'i,  gracilis,  sulcata,  elegans,  ccerulea, 
pctitpitrreana,  macrorrhiza,  It<dica,  secundiflora,  and  Taurica, 
Bleep  in  nearly  the  same  manner  as  just  described;  but  the 
bending  to  one  side  of  the  terminal  leaflet  is  apt  to  fail  unless 
the  plants  are  growing  vigorously.  With  M.  petitpkrreat,a  and 
sccundiflora  the  terminal  leaflet  was  rarely  seen  to  bend  to  one 
side.  In  young  plants  of  M.  Italica  it  bent  in  the  usual  manner, 
but  with  old  plants  in  full  flower,  growing  in  the  same  pot  and 
observed  at  the  same  hour,  viz.,  8.30  P.M.,  none  of  the  terminal 
leaflets  on  several  scores  of  leaves  had  bent  to  one  side,  though 
they  stood  vertically ;  nor  nad  the  two  lateral  leaflets,  though 
standing  vertically,  moved  towards  the  terminal  one.  At 
1030  P.M.,  and  again  one  hour  after  midnight,  the  terminal 
leaflets  had  become  very  slightly  bent  to  one  side,  and  the 
lateral  leaflets  had  moved  a  very  little  towards  the  terminal  one, 
so  that  the  position  of  the  leaflets  even  at  this  late  hour  was  far 
from  the  ordinary  one.  Again,  with  M.  Taurica  the  terminal 
leaflets  were  never  seen  to  bend  towards  either  of  the  two  lateral 
leaflets,  chough  these,  whilst  becoming  vertical,  had  bent  towards 
the  terminal  one.  The  sub-petiole  of  the  terminal  leaflet  in 
this  species  is  of  unusual  length,  and  if  the  leaflet  had  bent  to 
one  side,  its  upper  surface  could  have  come  into  contact  only 
with  the  apex  of  either  lateral  leaflet ;  and  this,  perhaps,  is  the 
meaning  of  the  loss  of  the  lateral  movement. 

The  cotyledons  do  not  sleep  at  night.  The  first  leaf  consists  of 
a  single  orbicular  leaflet,  which  twists  at  night  so  that  the  blade 
stands  vertically.  It  is  a  remarkable  fact  that  with  M.  Taurica, 
and  in  a  somewhat  less  degree  with  M.  macrorrhiza  and  petit- 
pierreana,  all  the  many  small  and  young  leaves  produced  during 


348  MODIFIED  CIRCUMNUTATION.  CHAP.  VII. 

the  early  spring  from  shoots  on  some  cut-down  plants  in  the 
greenhouse,  slept  in  a  totally  different  manner  from  the  normal 
one ;  for  the  three  leaflets,  instead  of  twisting  on  their  own  axes 
so  as  to  present  their  lateral  edges  to  the  zenith,  turned  upwards 
and  stood  vertically  with  their  apices  pointing  to  the  zenith. 
They  thus  assumed  nearly  the  same  position  as  in  the  allied 
genus  Trifolium ;  and  on  the  same  principle  that  embryological 
characters  reveal  the  lines  of  descent  in  the  animal  kingdom,  so 
the  movements  of  the  small  leaves  in  the  above  three  species  of 
Melilotus,  perhaps  indicate  that  this  genus  is  descended  from 
a  form  which  was  closely  allied  to  and  slept  like  a  Trifolium. 
Moreover,  there  is  one  species,  M.  messanensis,  the  leaves  of 
which,  on  full-grown  plants  between  2  and  3  feet  in  height, 
sleep  like  the  foregoing  small  leaves  and  like  those  of  a  Trifolium. 
We  were  so  much  surprised  at  this  latter  case  that,  until  the 
flowers  and  fruit  were  examined,  we  thought  that  the  seeds  of 
some  Trifolium  had  been  sown  by  mistake  instead  of  those  of  a 
Melilotus.  It  appears  therefore  probable  that  M.  messanensis 
has  either  retained  or  recovered  a  primordial  habit. 

The  circumnutation  of  a  leaf  of  M.  ojficinalis  was  traced, 
the  stem  being  left  free;  and  the  apex  of  the  terminal  leaflet 
described  three  laterally  extended  ellipses,  between  8  A.M.  and 
4  P.M.  ;  after  the  latter  hour  the  nocturnal  twisting  movement 
commenced.  It  was  afterwards  ascertained  that  the  above 
movement  was  compounded  of  the  circumnutation  of  the  stem 
on  a  small  scale,  of  the  main  petiole  which  moved  most,  and  of 
the  sub-petiole  of  the  terminal  leaflet.  The  main  petiole  of  a 
leaf  having  been  secured  to  a  stick,  close  to  the  base  of  the  sub- 
petiole  of  the  terminal  leaflet,  the  latter  described  two  small 
ellipses  between  10.30  A.M.,  and  2  P.M.  At  7.15  P.M.,  after  this 
same  leaflet  (as  well  as  another)  had  twisted  themselves  into 
their  vertical  nocturnal  position,  they  began  to  rise  slowly,  and 
continued  to  do  so  until  10.35  P.M.,  after  which  hour  they  were 
no  longer  observed. 

As  M.  messanensis  sleeps  in  an  anomalous  manner,  unlike  that 
of  any  other  species  in  the  genus,  the  circumnutation  of  a 
terminal  leaflet,  with  the  stem  secured,  was  traced  during  two 
days.  On  each  morning  the  leaflet  fell,  until  about  noon,  and 
then  began  to  rise  very  slowly ;  but  on  the  first  day  the  rising 
movement  was  interrupted  between  1  and  3  P.M.  by  the  formation 
of  a  laterally  extended  ellipse,  .and  on  the  second  day,  at  the 
same  time,  by  two  smaller  ellipses.  The  rising  movement  then 


CHAP.  VII.  SLEEP   OF   LEAVES.  349 

recommenced,  and  became  rapid  late  in  the  evening,  when 
the  leaflet  was  beginning  to  go  to  sleep.  The  awaking  or 
sinking  movement  had  already  commenced  by  6.45  A.M  on  both 
mornings. 

Trifolium  (Tribe  3).  —  The  nyctitropic  movements  of  11 
species  were  observed,  and  were  found  to  be  closely  similar.  If 
we  select  a  leaf  of  T.  repens  having  an  upright  petiole,  and  with 
the  three  leaflets  expanded  horizontally,  the  two  lateral  leaflets 
will  be  seen  in  the  evening  to  twist  and  approach  each  other, 
until  their  upper  surfaces  come  into  contact.  At  the  same  time 
they  bend  downwards  in  a  plane  at  right  angles  to  that  of  their 
former  position,  until  their  midribs  form  an  angle  of  about  45° 
with  the  upper  part  of  the  petiole.  This  peculiar  change  of 
position  requires  a  considerable  amount  of  torsion  in  the  pul- 
vinus.  The  terminal  leaflet  merely  rises  up  without  any  twist- 
Fig.  141. 


A.  B. 

Trifolium  repens :  A,  leaf  during  the  day  ;  B,  leaf  asleep  at  night. 

ing,  and  bends  over  until  it  rests  on  and  forms  a  roof  over  the 
edges  of  the  now  vertical  and  united  lateral  leaflets.  Thus  the 
terminal  leaflet  always  passes  through  an  angle  of  at  least  90°, 
generally  of  130°  or  140°,  and  not  rarely — as  was  often  observed 
with  T.  subterraneum — of  180°.  In  this  latter  case  the  terminal 
leaflet  stands  at  night  horizontally  (as  in  Fig.  141),  with  its 
lower  surface  fully  exposed  to  the  zenith.  Besides  the  difference 
in  the  angles,  at  which  the  terminal  leaflets  stand  at  night  in 
the  individuals  of  the  same  species,  the  degree  to  which  the 
lateral  leaflets  approach  each  other  often  likewise  differs. 

We  have  seen  that  the  cotyledons  of  some  species  and  not  of 
others  rise  up  vertically  at  night.  The  first  true  leaf  is  generally 
unifoliate  and  orbicular ;  it  always  rises,  and  either  stands  verti- 
cally at  night  or  more  commonly  bends  a  little  over  so  as  to  expose 
the  lower  surface  obliquely  to  the  zenith,  in  the  same  manner 
as  does  the  terminal  leaflet  of  the  mature  leaf.  But  it  does  not 
twist  itself  like  the  corresponding  first  simple  leaf  of  Melilotus. 


350  MODIFIED   CiKCUMNUTATION.  CHAP.  VII. 

With  T.  Pannonicum  the  first  true  leaf  was  generally  unifoliate,. 
but  sometimes  trifoliate,  or  again  partially  lobed  and  in  an 
intermediate  condition. 

Circumnutation. — Sachs  described  in  1863*  the  spontaneous 
up  and  down  movements  of  the  leaflets  of  T.  incarnatum,  when 
kept  in  darkness.  Pfeffer  made  many  observations  on  the 
similar  movements  in  T.  pratense.^  He  states  that  the  terminal 
leaflet  of  this  species,  observed  at  different  times,  passed  through 
angles  of  from  30°  to  120°  in  the  course  of  from  U  to  4h.  We 
observed  the  movements  of  T.  subterraneum,  resupinatu?n,  and 
repens. 

Trifolium  sulterranenm. — A  petiole  was  secured  close  to  the 
base  of  the  three  leaflets,  and  the  movement  of  the  terminal 
leaflet  was  traced  during  26!  h.,  as  shown  in  the  figure  on  the 
next  page. 

Between  6.45  A.M.  and  6  P.M.  the  apex  moved  3  times  up 
and  3  times  down,  completing  3  ellipses  in  11  h.  15  m.  The 
ascending  and  descending  lines  stand  nearer  to  one  another 
than  is  usual  with  most  plants,  yet  there  was  some  lateral 
motion.  At  6  P.M.  the  great  nocturnal  rise  commenced,  and 
on  the  next  morning  the  sinking  of  the  leaflet  was  continued 
until  8.30  A.M.,  after  which  hour  it  circumnutated  in  the  manner 
just  described.  In  the  figure  the  great  nocturnal  rise  and 
the  morning  fall  are  greatly  abbreviated,  from  the  want  of 
space,  and  are  merely  represented  by  a  short  curved  line.  The 
leaflet  stood  horizontally  when  at  a  point  a  little  beneath  the 
middle  of  the  diagram;  so  that  during  the  daytime  it  oscillated 
almost  equally  above  and  beneath  a  horizontal  position.  At 
8.30  A.M.  it  stood  48°  beneath  the  horizon,  and  by  ll.oO  A.M.  it 
had  risen  50°  above  the  horizon ;  so  that  it  passed  through  98° 
in  3  h.  By  the  aid  of  the  tracing  we  ascertained  that  the 
distance  travelled  in  the  3  h.  by  the  apex  of  this  leaflet  was 
1-03  inch.  If  we  look  at  the  figure,  and  prolong  upwards  in 
our  mind's  eye  the  short  curved  broken  line,  which  repre- 
sents the  nocturnal  course,  we  see  that  the  latter  movement  is 
merely  an  exaggeration  or  prolongation  of  one  of  the  diurnal 
ellipses.  The  same  leaflet  had  been  observed  on  the  previous 
day,  and  the  course  then  pursued  was  almost  identically  the 
same  as  that  here  described. 


*  'Flora,'  18H3,  p.  497. 

t  '  Die  Ft  riod.  Bewegungen,'  1875,  pp.  35,  52. 


OdAP.   VII. 


SLEEP  OF   LEAVES. 


351 


Fig.  142. 


•  Trifolium  resupinatum. — 'A  plant  left  entirely 
before  a  north-east  win- 
dow, in  such  a  position 
that  a  terminal  leaflet 
projected  at  right  angles 
to  the  source  of  the  light, 
the  sky  being  uniformly 
clouded  all  day.  The 
movements  of  this  leaflet 
were  traced  during  two 
days,  and  on  both  were 
closely  similar.  Those 
executed  on  the  second 
day  are  shown  in  Fig. 
143.  The  obliquity  of 
the  several  lines  is  due 
partly  to  the  manner  in 
which  the  leaflet  war 
viewed,  and  partly  to  its 
having  moved  a  little  to- 
wards the  light.  From 
7.50  A.M.  to  8.40  A.M.  the 
leaflet  fell,  that  is,  the 
awakening  movement  was 
continued.  It  then  rose 
and  moved  a  little  late- 
rally towards  the  light. 
At  12.30  it  retrograded, 
and  at  2.30  resumed  its 
original  course,  having 
thus  completed  a  small 
ellipse  during  the  middle 
of  the  day.  In  the  even- 
ing it  rose  rapidly,  and 
by  8  A.M.  on  the  following 
morning  had  returned  to 
exactly  the  same  spot  as 
on  the  previous  morning. 
The  line  representing  the 
nocturnal  course  ought 
to  be  .extended  much 
higher  up,  and  is  here 
abbreviated  into  a  short, 
16 


fret  was  placed 


352 


MODIFIED  CIRCUMNUTATION. 


CHAP.  VII. 


curved,  broken  line.     The  terminal  leaflet,  therefore,  of  this 
species  described  during  the  daytime  only  a  single  additional 

ellipse,  instead  of  two  ad- 
ditional 
case    of 


143. 


ones,  as  in  the 
T.  subterraneurn. 
But  we  should  remember 
that  it  was  shown  in  the 
fourth  chapter  that  the 
stem  circumnutates,  as  no 
doubt  does  the  main  petiole 
and  the  sub-petioles;  so 
that  the  movement  repre- 
sented in  fig.  143  is  a  com- 
pounded one.  We  tried 
to  observe  the  movements 
of  a  leaf  kept  during  the 
day  in  darkness,  but  it 

began  to  go  to  sleep  after 
Jnjoliiim   resupinatum :    circumnutatnon       r"  _        . 

and  nyctitropic  movements  of  the  ter-  *  n-    15  m-»  and  this  was 
rainal  leaflet  during  24  hours.  well  pronounced  after  4  h. 

30m. 

Trifolium  repens. — A  stem  was  secured  close  to  the  base  of 
a  moderately  old  leaf,  and  the  movement  of  the  terminal  leaflet 
was  observed  during  two  days.  This  case  is  interesting  solely 
from  the  simplicity  of  the  movements,  in  contrast  with  those  of 
the  two  preceding  species.  On  the  first  day  the  leaflet  fell 
between  8  A.M.  and  3  P.M.,  and  on  the  second  between  7  A.M. 
and  1  P.M.  On  both  days  the  descending  course  was  somewhat 
zigzag,  and  this  evidently  represents  the  circumnutating  move- 
ment of  the  two  previous  species  during  the  middle  of  the  day. 
After  1  P.M.,  Oct.  1st  (Fig.  144),  the  leaflet  began  to  rise,  but 
the  movement  was  slow  on  both  days,  both  before  and  after 
this  hour,  until  4  P.M.  The  rapid  evening  and  nocturnal  rise 
then  commenced.  Thus  in  this  species  the  course  during  24  h. 
consists  of  a  single  great  ellipse;  in  T.  resupinatum  of  two 
ellipses,  one  of  which  includes  the  nocturnal  movement  and  is 
much  elongated;  and  in  T.  suUerraneum  of  three  ellipses,  of 
which  the  nocturnal  one  is  likewise  of  great  length. 

Securigera  coronilla  (Tribe  4). — The  leaflets,  which  stand 
opposite  o«e  another  and  are  numerous,  rise  up  at  night,  come 
into  close  contact,  and  bend  backwards  at  a  moderate  angle 
towards  the  base  of  the  petiole. 


CHAP.  VII. 


SLEEP   OF  LEAVES. 


353 


Fig.  144. 


Lotus  (Tribe  4). — The  nyctitropic  movements  of  10  species 
in  this  genus  were  observed,  and  found  to  be  the  same.  The 
main  petiole  rises  a  little  at  night,  and 
the  three  leaflets  rise  till  they  become 
vertical,  and  at  the  same  time  approach 
each  other.  This  was  conspicuous  with 
L.  Jacobceus,  in  which  the  leaflets  are 
almost  linear.  In  most  of  the  species 
the  leaflets  rise  so  much  as  to  press 
against  the  stem,  and  not  rarely  they 
become  inclined  a  little  inwards  with 
their  lower  surfaces  exposed  obliquely 
to  the  zenith.  This  was  clearly  the 
case  with  L.  major,  as  its  petioles  are 
unusually  long,  and  the  leaflets  are  thus 
enabled  to  bend  further  inwards.  The 
young  leaves  on  the  summits  of  the 
stems  close  up  at  night  so  much,  as 
often  to  resemble  large  buds.  The 
stipule-like  leaflets,  which  are  often  of 
large  size,  rise  up  like  the  other  leaflets, 
and  press  against  the  stem  (Fig.  145). 
All  the  leaflets  of  L.  Gebelii,  and  pro- 
bably of  the  other  species,  are  provided 
at  their  bases  with  distinct  pulvini,  of 

a  yellowish  colour,  and  formed  of  very    m  ., 

J     Trifokum  repens  :  circum- 
small  cells.     The  circumnutation  of  a 

terminal  leaflet  of  L.  perigrinus  (with 

the  stem   secured)  was  traced  during 

two  days,  but  the  movement  was  so 

simple  that  it  is  not  worth  while  to 

give    the    diagram.      The    leaflet    fell 

slowly    from    the   early    morning    till 

about  1  P.M.    It  then  rose  gradually 

at  first,  but  rapidly  late  in  the  evening. 

Jt  occasionally  stood  still  for  about  20  m.  during  the  day,  and 

sometimes  zigzagged  a  little.     The  movement  of  one  of  the 

basal,  stipule-like  leaflets  was   likewise  traced  in  the  same 

manner  and  at  the  same  time,  and  its  course  was  closely  similar 

to  that  of  the  terminal  leaflet. 

In  Tribe  5  of  Bentham  and  Hooker,  the   sleep-movements 
of  species  in  12  genera  have  been  observed  by  ourselves  and 


nutation  and  nyctitropic 
movements  of  a  nearly 
full  -  grown  terminal 
leaflet,  traced  on  a  ver- 
tical glass  from  7  A.M. 
Sept.  30th  to  8  A.M.  Oct. 
1st.  Nocturnal  course, 
represented  by  curved 
broken  line,  much  ab- 
breviated. 


354 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VII 


others,  but  only  in  Eobiuia  with  any  care.  Psoraka  acavlis 
raises  its  three  leaflets  at  night ;  whilst  Amorpha  fruticosa* 
Dalea  alopecuroides,  and  Indigofera  tinctoria  depress  them. 
Duchartre  f  states  that  Tcphrosia  caribcea  is  the  sole  example 
of  "  folioles  couchees  le  long  du  petiole  et  vers  la  base ; "  but  a 

Fig.  145. 


B. 

Lotus  Cieticus :  A,  stem  with  leaves  awake  during  the  day ;  B,  with  leaves 
asleep  at  night.     SS,  stipule-like  leaflets. 

similar  movement  occurs,  as  we  have  already  seen,  and  shall 
again  see  in  other  cases.  Wistaria  Sinensis,  according  to 
Royer.J  "abaisse  les  folioles  qui  par  une  disposition  bizarre 
sont  inclinees  dans  la  meme  feuille,  les  sup6rieures  vers  le 


*  Ducharte,      « Elements     de 
Botanique,'  1867,  p.  349. 
t  Ibid.,  p.  347. 


J  'Ann.    des   Sciences,   Nats. 
Bot.'  (5th  series),  ix.  1808. 


CHAP.  VII.  SLEEP   OF   LEAVES.  355 

sommet,  les  inferieures  vers  la  base  du  petiole  comnmn ; "  but 
the  leaflets  on  a  young  plant  observed  by  us  in  the  green- 
house merely  sank  vertically  downwards  at  night.  The  leaflets 
are  raised  in  Sphcerophysa  salsola,  Colutea  arborea,  and  Astra- 
galus uliyinosus,  but  are  depressed,  according  to  Linnaeus,  in 
Gtycyrrhiza.  The  leaflets  of  Bobinia  psevdo-acacia  likewise  sink 
vertically  down  at  night,  but  the  petioles  rise  a  little,  viz.,  in 
one  case  3°,  and  in  another  4°.  The  circumnutating  move- 
ments of  a  terminal  leaflet  on  a  rather  old  leaf  were  traced 
during  two  days,  and  were  simple.  The  leaflet  fell  slowly,  in  a 
slightly  zigzag  line,  from  8  A.M.  to  5  P.M.,  and  then  more 
rapidly ;  by  7  A.M.  on  the  following  morning  it  had  risen  to  its 
diurnal  position.  There  was  only  one  peculiarity  in  the  move- 
ment, namely,  that  on  both  days  there  was  a  distinct  though 
small  oscillation  up  and  down  between  8.30  and  10  A.M.,  and 
this  irould  probably  have  been  more  strongly  pronounced  if 
the  leaf  had  been  younger. 

Coronilla  rosea  (Tribe  6). — The  leaves  bear  9  or  10  pairs  of 
opposite  leaflets,  which  during  the  day  stand  horizontally,  with 

Fig.  146. 


Coronilla  rosea :  leaf  asleep. 

their  midribs  at  right  angles  to  the  petiole.  At  night  they  rise 
up,  so  that  the  opposite  leaflets  come  nearly  into  contact,  and 
those  on  the  younger  leaves  into  close  contact.  At  the  same 
time  they  bend  back  towards  the  base  of  the  petiole,  until  their 
midribs  form  with  it  angles  of  from  40°  to  50°  in  a  vertical 
plane,  as  here  figured  (Fig.  146).  The  leaflets,  however,  some- 
times bend  so  much  back  that  their  midribs  become  parallel  to 
and  lie  on  the  petiole.  They  thus  occupy  a  reversed  position 
to  what  they  do  in  several  Leguminosse,  for  instance,  in  Mimosa 


356 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VII. 


pudica ;  but,  from  standing  further  apart,  they  do  not  overlap 
one  another  nearly  so  much  as  in  this  latter  plant.  The  main 
petiole  is  curved  slightly  downwards  during  the  day,  but 
straightens  itself  at  night.  In  three  cases  it  rose  from  3°  above 
the  horizon  at  noon,  to  9°  at  10  P.M.  ;  from  11°  to  33°;  and  from 
5°  to  33° — the  amount  of  angular  movement  in  this  latter  case 
amounting  to  28°.  In  several  other  species  of  Coronilla  the 
leaflets  showed  only  feeble  movements  of  a  similar  kind. 

Hedysarum  coronarium  (Tribe  6). — The  small  lateral  leaflets 
on  plants  growing  out  of  doors  rose  up  vertically  at  night,  but 
the  large  terminal  one  became  only  moderately  inclined.  The 
petioles  apparently  did  not  rise  at  all. 

Smithia  Pfundii  (Tribe  6).— The  leaflets  rise  up  vertically, 
and  the  main  petiole  also  rises  considerably. 

Arachis  hypogcea  (Tribe  6). — The  shape  of  a  leaf,  with  its  two 
pairs  of  leaflets,  is  shown  at  A  (Fig.  147) ;  and  a  leaf  asleep, 

Fig.  147. 


A  B. 

Arachis  hypogcea :  A,  leaf  during  the  day,  seen  from  vertically  above  ;  B, 
leaf  asleep,  seen  laterally ;  copied  from  a  photograph.  Figures  much 
reduced. 

traced  from  a  photograph  (made  by  the  aid  of  aluminium 
light),  is  given  at  B.  The  two  terminal  leaflets  twist  round  at 
night  until  their  blades  stand  vertically,  and  approach  each 
other  until  they  meet,  at  the  same  time  moving  a  little  upwards 
and  backwards.  The  two  lateral  leaflets  meet  each  other  in  the 
same  manner,  but  move  to  a  greater  extent  forwards,  that  is,  in 
a  contrary  direction  to  the  two  terminal  leaflets,  which  they 
partially  embrace.  Thus  all  four  leaflets  form  together  a  single 
packet,  with  their  edges  directed  to  the  zenith,  and  with  their 
lower  surfaces  turned  outwards.  On  a  plant  which  was  not 
growing  vigorously  the  closed  leaflets  seemed  too  heavy  for  the 


CHAP.  VII. 


SLEEP  OF  LEAVES. 


357 


Fig.  148. 


petioles  to  support  them  in  a  vertical  position,  so  that  each 
night  the  main  petiole  became  twisted,  and  all  the  packets  were 
extended  horizontally,  with  the  lower  surfaces  of  the  leaflets  on 
one  side  directed  to  the  zenith  in  a  most  anomalous  manner. 
This  fact  is  mentioned  solely  as  a  caution,  as  it  surprised  us 
greatly,  until  we  discovered  that  it  was  an  anomaly.  The 
petioles  are  inclined  upwards  during  the  day,  but  sink  at  night, 
so  as  to  stand  at  about  right  angles  with  the  stem.  The  amount 
of  sinking  was  measured  only  on  one  occasion,  and  found  to  be 
39°.  A  petiole  was  secured  to  a  stick  at  the  base  of  the  two 
terminal  leaflets,  and  the  circumnutating  movement  of  one  of 
these  leaflets  was  traced  from  6.40  A.M.  to  10.40  P.M.,  the  plant 
being  illuminated  from  above.  The  temperature  was  17° — 17^°  C., 
and  therefore  rather  too  low.  During  the  16  h.  the  leaflet  moved 
thrice  up  and  thrice  down,  and  as  the  ascending  and  descend- 
ing lines  did  not  coincide,  three  ellipses  were  formed. 

Desmodium  gyrans  (Tribe  6). — A  large  and  full-grown  leaf  of 
this  plant,  so  famous  for  the  spontaneous 
movements  of  the  two  little  lateral  leaflets, 
is  here  represented  (Fig.  148).  The  large 
terminal  leaflet  sleeps  by  sinking  vertically 
down,  whilst  the  petiole  rises  up.  The  coty- 
ledons do  not  sleep,  but  the  first-formed  leaf 
sleeps  equally  well  as  the  older  ones.  The 
appearance  presented  by  a  sleeping  branch 
and  one  in  the  day-time,  copied  from  two 
photographs,  are  shown  at  A  and  B  (Fig. 
149),  and  we  see  how  at  night  the  leaves  are 
crowded  together,  as  if  for  mutual  pro- 
tection, by  the  rising  of  the  petioles.  The 
petioles  of  the  younger  leaves  near  the  sum- 
mits of  the  shoots  rise  up  at  night,  so  as  to 
stand  vertical  and  parallel  to  the  stem ; 
whilst  those  on  the  sides  were  found  in  four 
cases  to  have  risen  respectively  46  £°,  36°, 
20°,  and  19 '5°  above  the  inclined  positions 
which  they  had  occupied  during  the  day. 
For  instance,  in  the  first  of  these  four  cases 
the  petiole  stood  in  the  day  at  23°,  and  at 
night  at  69-i°  above  the  horizon.  In  the 
evening  the  rising  of  the  petioles  is  almost 
completed  before  the  leaflets  sink  perpendicularly  downwards. 


Desmodium  gyrans: 
leaf  seen  from 
above,  reduced 
to  one-half  na- 
tural size.  The 
minute  stipules 
unusually  largft. 


358  MODIFIED   CIKCUMNUTATICXN  CHAP.  VII. 

Circumnutation.  —  The  circumnutating  movements  of  four 
young  shoots  were  observed  during  5  h.  15  m. ;  and  in  this  time 
each  completed  an  oval  figure  of  small  size.  The  main  petiole 
also  circumnutates  rapidly,  for  in  the  course  of  31m.  (temp, 
91°  F.)  it  changed  its  course  by  as  much  as  a  rectangle  six  times, 
describing  a  figure  which  apparently,  represented  two  ellipses. 


Fig.  149. 


Desmodium  gyrans :  A,  stem  during  the  day ;  B,  stem  with  leaves  asleep. 
Copied  from  a  photograph  ;  figures  reduced. 

The  movement  of  the  terminal  leaflet  by  means  of  its  sub- 
petiole  or  pulvinus  is  quite  as  rapid,  or  even  more  so,  than  that 
of  the  main  petiole,  and  has  much  greater  amplitude.  Pfenx-r 
has  seen*  these  leaflets  move  through  an  angle  of  8°  in  the 
course  of  from  10  to  30  seconds. 

A  fine,  nearly  full-grown  leaf  on  a  young  plant,  8  inches  in 
height,  with  the  stem  secured  to  a  stick  at  the  base  of  the  leaf, 
'vas  observed  from  8.30  A.M.  June  22nd  to  8  A.M.  June  24th. 


'  Die  Period.  Beweg.,'  p.  35. 


CHAP.  VII.  SLEEP   OF   LEAVES.  359 

In  the  diagram  given  on  the  next  page  (Fig.  150),  the  two 
curved  broken  lines  at  the  base,  which  represent  the  nocturnal 
courses,  ought  to  be  prolonged  far  downwards.  On  the  first 
day  the  leaflet  moved  thrice  down  and  thrice  up,  and  to  a  con- 
siderable distance  laterally;  the  course  was  also  remarkably 
crooked.  The  dots  were  generally  made  every  hour;  if  they 
had  been  made  every  few  minutes  all  the  lines  would  have  been 
zigzag  to  an  extraordinary  degree,  with  here  and  there  a  loop 
formed.  We  may  infer  that  this  would  have  been  the  case, 
because  five  dots  were  made  in  the  course  of  31m.  (between 
]  2.34  and  1.5  P.M.),  and  we  see  in  the  upper  part  of  the  diagram 
how  crooked  the  course  here  is ;  if  only  the  first  and  last  dots 
had  been  joined  we  should  have  had  a  straight  line.  Exactly 
the  same  fact  may  be  seen  in  the  lines  representing  the  course 
between  2.34  P.M.  and  3  P.M.,  when  six  intermediate  dots  were 
made ;  and  again  at  4.46  and  4.50.  But  the  result  was  widely 
different  after  6  P.M., — that  is,  after  the  great  nocturnal  descent 
had  commenced ;  for  though  nine  dots  were  then  made  in  the 
course  of  32  m.,  when  these  were  joined  (see  Figure)  the  line  thus 
formed  was  almost  straight.  The  leaflets,  therefore,  begin  to 
descend  in  the  afternoon  by  zigzag  lines,  but  as  soon  as  the 
descent  becomes  rapid  their  whole  energy  is  expended  in  thus 
moving,  and  their  course  becomes  rectilinear.  After  the  leaflets 
are  completely  asleep  they  move  very  little  or  not  at  all. 

Had  the  above  plant  been  subjected  to  a  higher  temperature 
than  67° — 70°  F.,  the  movements  of  the  terminal  leaflet  would 
probably  have  been  even  more  rapid  and  wider  in  extent  than 
those  shown  in  the  diagram ;  for  a  plant  was  kept  for  some  time 
in  the  hot-house  at  from  92° — 93°  F.,  and  in  the  course  of  35  m. 
the  apex  of  a  leaflet  twice  descended  and  once  ascended,  travelling 
over  a  space  of  1'2  inch  in  a  vertical  direction  and  of  '82  inch  in 
a  horizontal  direction.  Whilst  thus  moving  the  leaflet  also 
rotated  on  its  own  axis  (and  this  was  a  point  to  which  no  atten- 
tion had  been  before  paid),  for  the  plane  of  the  blade  differed  by 
41°  after  an  interval  of  only  a  few  minutes.  Occasionally  the 
leaflet  stood  still  for  a  short  time.  There  was  no  jerking  move- 
ment, which  is  so  characteristic  of  the  little  lateral  leaflets.  A 
sudden  and  considerable  fall  of  temperature  causes  the  terminal 
leaflet  to  sink  downwards ;  thus  a  cut-off  leaf  was  immersed  in 
water  at  95°  F.,  which  was  slowly  raised  to  103°  F.,  and  after- 
wards allowed  to  sink  to  70°  F.,  and  the  sub-petiole  of  the  ter- 
minal leaflet  then  curved  downwards.  The  water  was  afterwards 


360  MODIFIED   CIRCUMNUTATION. 

Fig.  150. 


CHAP.  VII. 


8°3(fu.m.22 


CHAP.  VII.  SLEEP  OF  LEAVES  361 

raised  to  120°  F.,  and  the  sub-petiole  straightened  itself.  Similar 
experiments  with  leaves  in  water  were  twice  repeated,  with 
nearly  the  same  result.  It  should  be  added,  that  water  raised 
to  even.  122°  F.  does  not  soon  kill  a  leaf.  A  plant  was  placed 
in  darkness  at  8.37  A.M.,  and  at  2  P.M.  (i.e.  after  5  h.  23  m.),  though 
the  leaflets  had  sunk  considerably,  they  had  by  no  means  ac- 
quired their  nocturnal  vertically  dependent  position.  Pfeffer,  on 
the  other  hand,  says  *  that  this  occurred  with  him  in  from  f  h. 
to  2  h. ;  perhaps  the  diiference  in  our  results  may  be  due  to 
the  plant  on  which  we  experimented  being  a  very  young  and 
vigorous  seedling. 

The  Movements  of  the  little  Lattral  Leaflets. — These  have  been  so 
often  described,  that  we  will  endeavour  to  be  as  brief  as  possible 
in  giving  a  few  new  facts  and  conclusions.  The  leaflets  some- 
times quickly  change  their  position  by  as  much  as  nearly  180° ; 
and  their  sub-petioles  can  then  be  seen  to  become  greatly  curved. 
They  rotate  on  their  own  axes,  so  that  their  upper  surfaces  are 
directed  to  all  points  of  the  compass.  The  figure  described  by 
the  apex  is  an  irregular  oval  or  ellipse.  They  sometimes  re- 
main stationary  for  a  period.  In  these  several  respects  there  is 
no  difference,  except  in  rapidity  and  extent,  between  their  move- 
ments and  the  lesser  ones  performed  by  the  large  terminal 
leaflet  whilst  making  its  great  oscillations.  The  movements  of 
the  little  leaflets  are  much  influenced,  as  is  well  known,  by 
temperature.  This  was  clearly  shown  by  immersing  leaves  with 
motionless  leaflets  in  cold  water,  which  was  slowly  raised  to 
103°  F.,  and  the  leaflets  then  moved  quickly,  describing  about  a 
dozen  little  irregular  circles  in  40  m.  By  this  time  the  water 
had  become  much  cooler,  and  the  movements  became  slower  or 
almost  ceased ;  it  was  then  raised  to  100°  F.,  and  the  leaflets 
again  began  to  move  quickly.  On  another  occasion  a  tuft  of 
fine  leaves  was  immersed  in  water  at  53°  F.,  and  the  leaflets 
were  of  course  motionless.  The  water  was  raised. to  99°,  and 
the  leaflets  soon  began  to  move ;  it  was  raised  to  105°,  and  the 
movements  became  much  more  rapid ;  each  little  circle  or  oval 
being  completed  in  from  1  m.  30  s.  to  1  m.  45  s.  There  was, 
however,  no  jerking,  and  this  fact  may  perhaps  be  attributed  to 
the  resistance  of  the  water. 

Sachs  states  that  the  leaflets  do  not  move  until  the  surround- 
ing air  is  as  high  as  71° — 72°  F.,  and  this  agrees  with  our 


*  « Die  Period.  Beweg.,'  p.  39. 


362    /  MODIFIED  CIRCUMNUTATION.  CHAP.  VII. 

experience  on  full-grown,  or  nearly  full-grown,  plants.  But  the 
leaflets  of  young  seedlings  exhibit  a  jerking  movement  at  much 
lower  temperatures.  A  seedling  was  kept  (April  16th)  in  a  room 
for  half  the  day  where  the  temperature  was  steady  at  64°  F., 
and  the  one  leaflet  which  it  bore  was  continually  jerking,  but 
not  so  rapidly  as  in  the  hot-house.  The  pot  was  taken  in  the 
evening  into  a  bed-room  where  the  temperature  remained  at 
02°  during  nearly  the  whole  night ;  at  10  and  11  P.M.  and  at 
1  A.M.  the  leaflet  was  still  jerking  rapidly ;  at  3.30  A  M.  it  was  not 
seen  to  jerk,  but  was  observed  during  only  a  short  time.  It  was, 
however,  now  inclined  at  a  much  lower  angle  than  that  occupied 
at  1  A.M.  At  6.30  A.M.  (temp.  61°  F.)  its  inclination  was  still 
less  than  before,  and  again  less  at  6.45  A.M;  by  7.40A.M.  it  had 
risen,  and  at  8.30A.M.  was  again  seen  to  jerk.  This  leaflet, 
therefore,  was  moving  during  the  whole  night,  and  the  move- 
ment was  by  jerks  up  to  1  A.M.  (and  possibly  later)  and  again  at 
8.30AM.,  though  the  temperature  was  only  61°  to  62°  F.  We 
must  therefore  conclude  that  the  lateral  leaflets  produced  by 
young  plants  differ  somewhat  in  constitution  from  those  on 
older  plants. 

In  the  large  genus  Desmodium  by  far  the  greater  number 
of  the  species  are  trifoliate ;  but  some  are  unifoliate,  and  even 
the  same  plant  may  bear  uni-  and  trifoliate  leaves.  In  most 
of  the  species  the  lateral  leaflets  are  only  a  little  smaller  than 
the  terminal  one.  Therefore  the  lateral  leaflets  of  D.  gyrans 
(see  former  Fig.  148)  must  be  considered  as  almost  rudi- 
mentary. They  are  also  rudimentary  in  function,-  if  this  ex- 
pression may  be  used ;  for  they  certainly  do  not  sleep  like  the 
full-sized  terminal  leaflets.  It  is,  however,  possible  that  the 
sinking  down  of  the  leaflets  between  1  A.M.  and  6.45  A.M  ,  as 
above  described,  may  represent  sleep.  It  is  well  known  that 
the  leaflets  go  on  jerking  during  the  early  part  of  the  night; 
but  my  gardener  observed  (Oct.  13th)  a  plant  in  the  hot-house 
between  5  and  5.30  A.M.,  the  temperature  having  been  kept  up 
to  82°  F.,  and  found  that  all  the  leaflets  were  inclined,  but  he 
saw  no  jerking  movement  until  6.55  A.M.,  by  which  time  the 
terminal  leaflet  had  risen  and  was  awake.  Two  days  after- 
wards (Oct.  15th)  the  same  plant  was  observed  by  him  at 
4.47  A.M.  (temp.  77°  F.),  and  he  found  that  the  large  terminal 
leaflets  were  awake,  though  not  quite  horizontal ;  and  the  only 
cause  which  we  could  assign  for  this  anomalous  wakefulness  was 
that  the  plant  had  been  kept  for  experimental  purposes  during 


CHAP.  VII.  SLEEP   OF   LEAVES.  363 

the  previous  day  at  an  unusually  high  temperature ;  the  little 
lateral  leaflets  were  also  jerking  at  this  hour,  but  whether 
there  was  any  connection  between  this  latter  fact  and  the  sub- 
horizontal  position  of  the  terminal  leaflets  we  do  not  know. 
Anyhow,  it  is  certain  that  the  lateral  leaflets  do  not  sleep  like 
the  terminal  leaflets;  and  in  so  far  they  may  be  seid  to  be 
in  a  functionally  rudimentary  condition.  They  are  in  a  similar 
condition  in  relation  to  irritability;  for  if  a  plant  be  shaken 
or  syringed,  the  terminal  leaflets  sink  down  to  about  45°  be- 
neath the  horizon ;  but  we  could  never  detect  any  effect  thus 
produced  on  the  lateral  leaflets;  yet  we  are  not  prepared  to 
assert  positively  that  rubbing  or  pricking  the  pulvinus  produces 
no  effect. 

As  in  the  case  of  most  rudimentary  organs,  the  leaflets  are 
variable  in  size ;  they  often  depart  from  their  normal  position 
and  do  not  stand  opposite  one  another ;  and  one  of  the  two  is 
frequently  absent.  This  absence  appeared  in  some,  but  not  in 
all  the  cases,  to  be  due  to  the  leaflet  having  become  completely 
confluent  with  the  main  petiole,  as  might  be  inferred  from  the 
presence  of  a  slight  ridge  along  its  upper  margin,  and  from  the 
course  of  the  vessels.  In  one  instance  there  was  a  vestige  of 
the  leaflet,  in  the  shape  of  a  minute  point,  at  the  further  end  of  the 
ridge.  The  frequent,  sudden,  and  complete  disappearance  of  one 
or  both  of  the  rudimentary  leaflets  is  a  rather  singular  fact ;  but 
it  is  a  much  more  surprising  one  that  the  leaves  which  are  first 
developed  on  seedling  plants  are  not  provided  with  them.  Thus, 
on  one  seedling  the  seventh  leaf  above  the  cotyledons  was  the 
first  which  bore  any  lateral  leaflets,  and  then  only  a  single  one. 
On  another  seedling,  the  eleventh  leaf  first  bore  a  leaflet ;  of  the 
nine  succeeding  leaves  five  bore  a  single  lateral  leaflet,  and 
four  bore  none  at  all;  at  last  a  leaf,  the  twenty-first  above  the 
cotyledons,  was  provided  with  two  rudimentary  lateral  leaflets. 
From  a  widespread  analogy  in  the  animal  kingdom,  it  might 
have  been  expected  that  these  rudimentary  leaflets  would  have 
been  better  developed  and  more  regularly  present  on  very  young 
than  on  older  plants.  But  bearing  in  mind,  firstly,  that  long- 
lost  characters  sometimes  reappear  late  in  life,  and  secondly, 
that  the  species  of  Desmodium  are  generally  trifoliate,  but  that 
some  are  unifoliate,  the  suspicion  arises  that  D.  gyrans  is 
descended  from  a  unifoliate  species,  and  that  this  was  descended 
from  a  trifoliate  one ;  for  in  this  case  both  the  absence  of  the 
little  lateral  leaflets  on  very  young  seedlings,  and  their  sub- 


364  MODIFIED  CIRCUMNUTATION.  CHAP.  VII. 

sequent  appearance,  may  be  attributed  to  reversion  to  more  or 
less  distant  progenitors.* 

No  one  supposes  that  the  rapid  movements  of  the  lateral 
leaflets  of  D.  gyrans  are  of  any  use  to  the  plant;  and  why 
they  should  behave  in  this  manner  is  quite  unknown.  We 
imagined  that  their  power  of  movement  might  stand  in  some 
relation  with  their  rudimentary  condition,  and  therefore  ob- 
served the  almost  rudimentary  leaflets  of  Mimosa  albida  vel 
sensitiva  (of  which  a  drawing  will  hereafter  be  given,  Fig.  159) ; 
but  they  exhibited  no  extraordinary  movements,  and  at  night 
they  went  to  sleep  like  the  full-sized  leaflets.  There  is,  how- 
ever, this  remarkable  difference  in  the  two  cases ;  in  Desmo- 
dium  the  pulvinus  of  the  rudimentary  leaflets  has  not  been 
reduced  in  length,  in  correspondence  with  the  reduction  of  the 
blade,  to  the  same  extent  as  has  occurred  in  the  Mimosa ;  and  it 
is  on  the  length  and  degree  of  curvature  of  the  pulvinus  that  the 
amount  of  movement  of  the  blade  depends.  Thus,  the  average 
length  of  the  pulvinus  in  the  large  terminal  leaflets  of  Desmo- 
diurn  is  3  mm.,  whilst  that  of  the  rudimentary  leaflets  is  2'86  mm. ; 
so  that  they  differ  only  a  little  in  length.  But  in  diameter  they 
differ  much,  that  of  the  pulvinus  of  the  little  leaflets  being  only 
0'3  mm.  to  0'4  mm.;  whilst  that  of  the  terminal  leaflets  is 
1'33  mm.  If  we  now  turn  to  the  Mimosa,  we  find  that  the 
average  length  of  the  pulvinus  of  the  almost  rudimentary 
leaflets  is  only  0*466  mm.,  or  rather  more  than  a  quarter  of  the 
length  of  the  pulvinus  of  the  full-sized  leaflets,  namely,  1  •  66  mm. 
In  this  small  reduction  in  length  of  the  pulvinus  of  the  rudi- 
mentary leaflets  of  Desmodium,  we  apparently  have  the  proxi- 
mate cause  of  their  great  and  rapid  circumnutating  movement, 
in  contrast  with  that  of  the  almost  rudimentary  leaflets  of  the 
Mimosa.  The  small  size  and  weight  of  the  blade,  and  the  little 
resistance  opposed  by  the  air  to  its  movement,  no  doubt  also  come 
into  play ;  for  we  have  seen  that  these  leaflets  if  immersed  in 
water,  when  the  resistance  would  be  much  greater,  were  pre- 
vented from  jerking  forwards.  "Why,  during  the  reduction  of 
the  lateral  leaflets  of  Desmodium,  or  during  their  reappearance 
— if  they  owe  their  origin  to  reversion — the  pulvinus  should 
have  been  so  much  less  affected  than  the  blade,  whilst  with  the 


*  Desmodium  vespertilionis  is  rudimentary  lateral  leaflets.  Du- 
closely  allied  to  D.  gyrans,  and  chartre,  '  Elements  de  Botanique,' 
it  seems  only  occasionally  to  bear  1867,  p.  353. 


CHAP.  V  tl. 


SLEEP   OF   LEAVES. 


365 


Mimosa  the  pulvinus  has  been  greatly  reduced,  we  do  not 
know.  Nevertheless,  it  deserves  notice  that  the  reduction  of 
the  leaflets  in  these  two  genera  has  apparently  been  effected  by 
a  different  process  and  for  a  different  end ;  for  with  the  Mimosa 
the  reduction  of  the  inner  and  basal  leaflets  was  necessary  from 
the  want  of  space;  but  no  such  necessity  exists  with  Desmo- 
dium,  and  the  reduction  of  its  lateral  leaflets  seems  to  have 
been  due  to  the  principle  of  compensation,  in  consequence  of 
the  great  size  of  the  terminal  leaflet. 

Uraria  (Tribe  6)  and  Centrosema  (Tribe  8).— The  leaflets  of 
Uraria  lagopus  and  the  leaves  of  a  Centrosema  from  Brazil 
both  sink  vertically  down  at  night.  In  the  latter  plant  the 
petiole  at  the  same  time  rose  16^°. 

Amphicarpcea  monoica  (Tribe  8). — The  leaflets  sink  down  ver- 
tically at  night,  and  the  petioles  likewise  fall  considerably. 

Fig.  151. 


Amphicarpcea  monoica :  circumnutation  and  nyctitropic  movement  of  leaf 
during  48  h. ;  its  apex  9  inches  from  the  vertical  glass.  Figure  reduced 
to  one-third  of  original  scale.  Plant  illuminated  from  above;  temp. 
17£°-18£°  C. 

A  petiole,  which  was  carefully  observed,  stood  during  the  day 
25°  above  the  horizon  and  at  night  32°  below  it;  it  therefore 
fell  57°.  A  filament  was  fixed  transversely  across  the  terminal 
leaflet  of  a  fine  young  leaf  (2J  inches  in  length  including  the 


366  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

petiole),  and  the  movement  of  the  whole  leaf  was  traced  on  a 
vertical  glass.  This  was  a  bad  plan  in  some  respects,  because 
the  rotation  of  the  leaflet,  independently  of  its  rising  or  falling, 
raised  and  depressed  the  filament ;  but  it  was  the  best  plan  for 
our  special  purpose  of  observing  whether  the  leaf  moved  much 
after  it  had  gone  to  sleep.  The  plant  had  twined  closely  round 
a  thin  stick,  so  that  the  circumnutation  of  the  stem  was  pre- 
vented. The  movement  of  the  leaf  was  traced  during  48  h., 
from  9  A.M.  July  10th  to  9  A.M.  July  12th.  In  the  figure  given 
(Fig.  151)  we  see  how  complicated  its  course  was  on  both  days : 
during  the  second  day  it  changed  its  course  greatly  13  times. 
The  leaflets  began  to  go  to  sleep  a  little  after  6  P.M.,  and  by 
7.15  P.M.  hung  vertically  down  and  were  completely  asleep; 
but  on  both  nights  they  continued  to  move  from  7.15  P.M. 
to  10.40  and  10.50  P.M.,  quite  as  much  as  during  the  day ;  and 
this  was  the  point  which  we  wished  to  ascertain.  We  see  in 
the  figure  that  the  great  sinking  movement  late  in  the  evening 
does  not  differ  essentially  from  the  circumnutation  during 
the  day. 

Glycine  Idtpida  (Tribe  8).— The  three  leaflets  sink  vertically 
down  at  night. 

Erythrina  (Tribe  8). — Five  species  were  observed,  and  the 
leaflets  of  all  sank  vertically  down  at  night ;  with  E.  caffra  and 
with  a  second  unnamed  species,  the  petioles  at  the  same  time 
rose  slightly.  The  movements  of  the  terminal  leaflet  of  E.  crista- 
galli  (with  the  main  petiole  secured  to  a  stick)  were  traced 
from  6.40  A.M.,  June  8th,  to  8  A.M.  on  the  10th.  In- order  to 
observe  the  nyctitropic  movements  of  this  plant,  it  is  necessary 
that  it  should  have  grown  in  a  warm  greenhouse,  for  out  of 
doors  in  our  climate  it  does  not  sleep.  We  see  in  the  tracing 
(Fig.  152)  that  the  leaflet  oscillated  twice  up  and  down  between 
early  morning  and  noon ;  it  then  fell  greatly,  afterwards  rising 
till  3  P.M.  At  this  latter  hour  the  great  nocturnal  fall  com- 
menced. On  the  second  day  (of  which  the  tracing  is  not  given) 
there  was  exactly  the  same  double  oscillation  before  noon,  but 
only  a  very  small  one  in  the  afternoon.  On  the  third  morning 
the  leaflet  moved  laterally,  which  was  due  to  its  beginning  to 
assume  an  oblique  position,  as  seems  invariably  to  occur  with 
the  leaflets  of  this  species  as  they  grow  old.  On  both  nights  after 
the  leaflets  were  asleep  and  hung  vertically  down,  they  continued 
to  move  a  little  both  up  and  down,  and  from  side  to  side. 

Erythrina  caffra. — A  filament  was  fixed  transversely  across 


CHAP.  VII. 


SLEEP   OF   LEAVES. 


367 


a  terminal  leaflet,  as  we  wished 
to  observe  its  movements  when 
asleep.  The  plant  was  placed 
in  the  morning  of  June  10th 
under  a  skylight,  where  the 
light  was  not  bright ;  and  we 
do  not  know  whether  it  was 
owing  to  this  cause  or  to  the 
plant  having  been  disturbed, 
but  the  leaflet  hung  vertically 
down  all  day;  nevertheless  it 
circumnutated  in  this  posi- 
tion, describing  a  figure  which 
represented  two  irregular  el- 
lipses. On  the  next  day  it 
circumnutated  in  a  greater 
degree,  describing  four  irre- 
.gular  ellipses,  and  by  3  P.M. 
had  risen  into  a  horizontal  po- 
sition. By  7.15  P.M.  it  was 
asleep  and  vertically  depen- 
dent, but  continued  to  circum- 
nutate  as  long  as  observed, 
until  11  P.M. 

Erythrina  corcdlodendion. — 
The  movements  of  a  terminal 
leaflet  were  traced.  During 
the  second  day  it  oscillated 
four  times  up  and  four  times 
down  between  8  A.M.  and  4 
P.M.,  after  which  hour  the  great 
nocturnal  fall  commenced.  On 
the  third  day  the  movement 
was  equally  great  in  ampli- 
tude, but  was  remarkably 
simple,  for  the  leaflet  rose  in 
an  almost  perfectly  straight 
line  from  6.50  A.M.  to  3  P.M., 
and  then  sank  down  in  an 
equally  straight  line  until 
vertically  dependent  and 
asleep. 


Fig.  152. 


Krythrina  crista-galli:  circum nuta- 
tion and  nycti tropic  movement 
of  terminal  leaflet,  3|  inches  in 
length,  traced  during  25  h. ;  apex 
of  leaf  3  J  inches  from  the  vertical 
glass.  Figure  reduced  to  one-half 
of  original  scale.  Plant  illumi- 
nated from  above:  temp.  17A°— 
18£°  C. 


368  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

Apios  tuber osa  (Tribe  8). — The  leaflets  sink  vertically  down 
at  night. 

Phaseolus  vulyaris  (Tribe  8). — The  leaflets  likewise  sink  verti- 
cally down  at  night.  In  the  greenhouse  the  petiole  of  a  young 
leaf  rose  16°,  and  that  of  an  older  leaf  10°  at  night.  With 
plants  growing  out  of  doors  the  leaflets  apparently  do  not  sleep 
until  somewhat  late  in  the  season,  for  on  the  nights  of  July  llth 
and  12th  none  of  them  were  asleep ;  whereas  on  the  night  of 
August  15th  the  same  plants  had  most  of  their  leaflets  verti- 
cally dependent  and  asleep.  With  Ph.  caracalla  and  Hernan- 
dtsii,  the  primary  unifoliate  leaves  and  the  leaflets  of  the 
secondary  trifoliate  leaves  sink  vertically  down  at  night.  This 
holds  good  with  the  secondary  trifoliate  leaves  of  Ph.  Eox- 
burghii,  but  it  is  remarkable  that  the  primary  unifoliate  leaves, 
which  are  much  elongated,  rise  at  night  from  about  20°  to 
about  60°  above  the  horizon.  With  older  seedlings,  however, 
having  the  secondary  leaves  just  developed,  the  primary  leaves 
stand  in  the  middle  of  the  day  horizontally,  or  are  deflected 
a  little  beneath  the  horizon.  In  one  such  case  the  primary 
leaves  rose  from  26°  beneath  the  horizon  at  noon,  to  20°  above 
it  at  10  P.M.;  whilst  at  this  same  hour  the  leaflets  of  the 
secondary  leaves  were  vertically  dependent.  Here,  then,  we 
have  the  extraordinary  case  of  the  primary  and  secondary 
leaves  on  the  same  plant  moving  at  the  same  time  in  opposite 
directions. 

We  have  now  seen  that  the  leaflets  in  the  six  genera  of  Pha- 
seolese  observed  by  us  (with  the  exception  of  the  primary  leaves 
of  Phaseolus  Roxburghii)  all  sleep  in  the  same  manner,  namely, 
by  sinking  vertically  down.  The  movements  of  the  petioles 
were  observed  in  only  three  of  these  genera.  They  rose  in 
Centrosema  and  Phaseolus,  and  sunk  in  Amphicarpaea. 

Sophora  chrysophylla  (Tribe  10). — The  leaflets  rise  at  night, 
and  are  at  the  same  time  directed  towards  the  apex  of  the  leaf, 
as  in  Mimosa  pudica. 

Ccesalpinia,  Hcematoxylon,  Gleditscliia ,  Poinciana. — The  leaflets 
of  two  species  of  Csesalpinia  (Tribe  13)  rose  at  night.  With 
Hcematoxylon  Campechianum  (Tribe  13)  the  leaflets  move  for- 
wards at  night,  so  that  their  midribs  stand  parallel  to  the 
petiole,  and  their  now  vertical  lower  surfaces  are  turned  out- 
wards (Fig.  153).  The  petiole  sinks  a  little.  In  Gleditschia,  if 
we  understand  correctly  Duchartre's  description,  and  in  Poin- 


CHAP.  VII. 


SLEEP  OF  LEAVES. 


369 


ciana  Qilliesii  (both  belonging  to  Tribe  13),  the  leaves  behave 
in  the  same  manner. 

Fig.  153. 


A.  B. 

Ftematoxylon  Campechianum  :  A,  branch  during  daytime  ;  B,  branch  with 
leaves  asleep,  reduced  to  two-thirds  of  natural  scale. 

Cassia  (Tribe  14). — The  nyctitropic  movements  of  the  leaves 
in  many  species  in  this  genus  are  closely  alike,  and  are  highly 
complex.  They  were  first  briefly  described  by  Linnaeus,  and  since 
by  Duchartre.  Our  observations  were  made  chiefly  on  0.  flori- 
bunda  *  and  corymbosa,  but  several  other  species  were  casually 
observed.  The  horizontally  extended  leaflets  sink  down  verti- 
cally at  night ;  but  not  simply,  as  in  so  many  other  genera,  for 
each  leaflet  rotates  on  its  own  axis,  so  that  its  lower  surface 
faces  outwards.  The  upper  surfaces  of  the  opposite  leaflets  are 
thus  brought  into  contact  with  one  another  beneath  the  petiole, 
and  are  well  protected  (Fig.  154).  The  rotation  and  other  move- 
ments are  effected  by  means  of  a  well-developed  pulvinus  at  the 
base  of  each  leaflet,  as  could  be  plainly  seen  when  a  straight 
narrow  black  line  had  been  painted  along  it  during  the  day. 
The  two  terminal  leaflets  in  the  daytime  include  rather  less  than 
a  right  angle :  but  their  divergence  increases  greatly  whilst  they 


*  I  am  informed  by  Mr.  Dyer 
that  Mr.  Bentham  believes  that 
C.  floribunda  (a  common  green- 
house bush)  is  a  hybrid  raised  in 
France,  and  that  it  comes  very 


near  to  C.  Icevigata.  It  is  no  doubt 
the  same  as  the  form  described  by 
Lindley  (<  Bot.  Keg.,'  Tab.  1422) 
as  C.  Herbertiana. 


370  «   MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

sink  downwards  and  rotate,  so  that  they  stand  laterally  at  night, 
as  may  be  seen  in  the  figure.  Moreover,  they  move  somewhat 
backwards,  so  as  to  point  towards  the  base  of  the  petiole. 


154. 


Cassia  corymbosa :  A,  plant  during  day  ;  B,  same  plant  at  night. 
Both  figures  copied  from  photographs. 

Ill  one    instance  we  found  that  the  midrib   of  a   terminal 
leaflet  formed  at  night  an  angle  of  36°,  with  a  line  dropped 


CHAP.  VII. 


SLEEP   OF   LEAVES. 


371 


perpendicularly  from  the  end  of  the  petiole.  The  second  pair 
of  leaflets  likewise  moves  a  little  backwards,  but  less  than  the 
terminal  pair ;  and  the  third  pair  moves  vertically  downwards, 
or  even  a  little  forwards.  Thus  all  the  leaflets,  in  those  species 
which  bear  only  3  or  4  pairs,  tend  to  form  a  single  packet,  with 
their  upper  surfaces  in  contact,  and  their  lower  surfaces  turned 
outwards.  Lastly,  the  main  petiole  rises  at  night,  but  with 
leaves  of  different  ages  to  very  different  degrees,  namely,  some 
rose  through  an  angle  of  only  12°,  and  others  as  much  as  41°. 

Cassia  calliantha. — The  leaves  bear  a  large  number  of  leaflets, 
which  move  at  night  in  nearly  the  same  manner  as  just 
described;  but  the  petioles  apparently  do  not  rise,  and  one 
which  was  carefully  observed  certainly  fell  3°. 

Cassia  pubescens.  —  The  chief  difference  in  the  nyctitropic 

Fig.  155. 


Cassia  pubescens :  A,  Tipper  part  of  plant  during  the  day  ;  B,  same  plant 
at  night.     Figures  reduced  from  photographs. 

movements  of  this  species,  compared  with  those  of  the  former 
species,  consists  in  the  leaflets  not  rotating  nearly  so  much; 


372  MODIFIED  CIECUMNUTATION.  CHAP.  VIL 

therefore  their  lower  surfaces  face  but  little  outwards  at  night. 
The  petioles,  which  during  the  day  are  inclined  only  a  little 
above  the  horizon,  rise  at  night  in  a  remarkable  manner,  and 
stand  nearly  or  quite  vertically.  This,  together  with  the 
dependent  position  of  the  leaflets,  makes  the  whole  plant  won- 
derfully compact  at  night.  In  the  two  foregoing  figures,  copied 
from  photographs,  the  same  plant  is  represented  awake  and 
asleep  (Fig.  155),  and  we  see  how  different  is  its  appearance. 

Cassia  mimosoides. — At  night  the  numerous  leaflets  on  each 
leaf  rotate  on  their  axes,  and  their  tips  move  towards  the  apex 
of  the  leaf;  they  thus  become  imbricated  with  their  lower 
surfaces  directed  upwards,  and  with  their  midribs  almost 
parallel  to  the  petiole.  Consequently,  this  species  differs  from 
all  the  others  seen  by  us,  with  the  exception  of  the  following 
one,  in  the  leaflets  not  sinking  down  at  night.  A  petiole,  the 
movement  of  which  was  measured,  rose  8°  at  night. 

Cassia  Bardayana. — The  leaflets  of  this  Australian  species  are 
numerous,  very  narrow,  and  almost  linear.  At  night  they  rise  up 
a  little,  and  also  move  towards  the  apex  of  the  leaf.  For  instance, 
two  opposite  leaflets  which  diverged  from  one  another  during 
the  day  at  an  angle  of  104°,  diverged  at  night  only  72° ;  so  that 
each  had  risen  16°  above  its  diurnal  position.  The  petiole  of  a 
young  leaf  rose  at  night  34°,  and  that  of  an  older  leaf  19°. 
Owing  to  the  slight  movement  of  the  leaflets  and  the  consider- 
able movement  of  the  petiole,  the  bush  presents  a  different 
appearance  at  night  to  what  it  does  by  day;  yet  the. leaves  can 
hardly  be  said  to  sleep. 

The  circumnutating  movements  of  the  leaves  of  C.  floribunda, 
calhantha,  and  pubescens  were  observed,  each  during  three  or  four 
days ;  they  were  essentially  alike,  those  of  the  last-named  species 
being  the  simplest.  The  petiole  of  C.  floribunda  was  secured  to 
a  stick  at  the  base  of  the  two  terminal  leaflets,  and  a  filament 
was  fixed  along  the  midrib  of  one  of  them.  Its  movements. were 
traced  from  1  P.M.  on  August  13th  to  8.30  A.M.  17th ;  but  those 
during  the  last  2  h.  are  alone  given  in  Fig.  156.  From  8  A.M.  on 
each  day  (by  which  hour  the  leaf  had  assumed  its  diurnal  posi- 
tion) to  2  or  3  P.M.,  it  either  zigzagged  or  circumnutated  over 
nearly  the  same  small  space ;  at  between  2  and  3  P.M.  the  great 
evening  fall  commenced.  The  lines  representing  this  fall  and 
the  early  morning  rise  are  oblique,  owing  to  the  peculiar  manner 
in  which -the  leaflets  sleep,  as  already  described.  After  the 
leaflet  was  asleep  at  6  P.M.,  and  whilst  the  glass  filament  hung 


CHAP.  VII. 


SLEEP  OF  LEAVES. 


373 


perpendicularly  down,  the  movement  of  its  apex  was  traced 
until  10.30  P.M.;  and  during  this  whole  time  it  swayed  from 
side  to  side,  completing  more  than  one  ellipse. 

Bauhinia    (Tribe    15).—  Fig.  156 

The  nyctitropic  movements 
of  four  species  were  alike, 
and  were  highly  peculiar. 
A  plant  raised  from  seed 
sent  us  from  South  Brazil 
by  Fritz  Miiller,  was  more 
especially  observed.  The 
leaves  are  large  and  deeply 
notched  at  their  ends.  At 
night  the  two  halves  rise 
up  and  close  completely 
together,  like  the  opposite 
leaflets  of  many  Legumi- 
nosae.  With  very  young 
plants  the  petioles  rise  con- 
siderably at  the  same  time ; 
one,  which  was  inclined  at 
noon  45°  above  the  hori- 
zon, at  night  stood  at  75° ; 
it  thus  rose  30°;  another 
rose  34°.  Whilst  the  two 
halves  of  the  leaf  are  closing, 
the  midrib  at  first  sinks 
vertically  downwards  and 
afterwards  bends  back-  . 

wards,  so  as  to  pass  close  r 

along  .one  side  of  its  own 
upwardly  inclined  petiole;  7 

the  midrib  being  thus  di- 
rected towards  the  stem  or  /  y. 
axis  of  the  plant.  The  angle  /  / 
which  the  midrib  formed  //' 
with  the  horizon  was  mea-  // 
sured  in  one  case  at  dif- 
ferent hours :  at  noon  it  stood  horizontally ;  late  in  the  even- 
ing it  depended  vertically ;  then  rose  to  the  opposite  side,  and 
at  10.15  P.M.  stood  at  only  27°  beneath  the  horizon,  being 
directed  towards  the  stern.  It  had  thus  travelled  through  153°. 


374  MODIFIED  CIBCUMNUTATION.          CHAP.  VII. 

Owing  to  this  movement — to  the  leaves  being  folded — and  to 
the  petioles  rising,  the  whole  plant  is  as  much  more  compact  at 
night  than  during  the  day,  as  a  fastigiate  Lombardy  poplar  is 
compared  with  any  other  species  of  poplar.  It  is  remarkable 
that  when  our  plants  had  grown  a  little  older,  viz.,  to  a  height 
of  2  or  3  feet,  the  petioles  did  not  rise  at  night,  and  the  midribs 
of  the  folded  leaves  were  no  longer  bent  back  along  one  side  of 
the  petiole.  We  have  noticed  in  some  other  genera  that  the 
petioles  of  very  young  plants  rise  much  more  at  night  than  do 
those  of  older  plants. 

Tamarindus  Indica  (Tribe  16).— The  leaflets  approach  or 
meet  each  other  at  night,  and  are  all  directed  towards  the  apex 
of  the  leaf.  They  thus  become  imbricated  with  their  midribs 
parallel  to  the  petiole.  The  movement  is  closely  similar  to 
that  of  Hsematoxylon  (see  former  Fig.  153),  but  more  striking 
from  the  greater  number  of  the  leaflets. 

Adenanthera,  Prosopis,  and  Neptunia  (Tribe  20). — With  Ade- 
nantkera  pavonia  the  leaflets  turn  edgeways  and  sink  at  night. 
In  Prosopis  they  turn  upwards.  With  Neptunia  oleracea  the 
leaflets  on  the  opposite  sides  of  the  same  pinna  come  into 
contact  at  night  and  are  directed  forwards.  The  pinnae  them- 
selves move  downwards,  and  at  the  same  time  backwards  or 
towards  the  stem  of  the  plant.  The  main  petiole  rises. 

Mimosa  pudica  (Tribe  20). — This  plant  has  been  the  subject  of 
innumerable  observations;  but  there  are  some  points  in  rela- 
tion to  our  subject  which  have  not  been  sufficiently  attended 
to.  At  night,  as  is  well  known,  the  opposite  leaflets  come  into 
contact  and  point  towards  the  apex  of  the  leaf ;  they  thus  be- 
come neatly  imbricated  with  their  upper  surfaces  protected.  The 
four  pinnae  also  approach  each  other  closely,  and  the  whole  leaf 
is  thus  rendered  very  compact.  The  main  petiole  sinks  down- 
wards during  the  day  till  late  in  the  evening,  and  rises  until 
very  early  in  the  morning.  The  stem  is  continually  circumnu- 
tating  at  a  rapid  rate,  though  not  to  a  wide  extent.  Some  very 
young  plants,  kept  in  darkness,  were  observed  during  two  days, 
and  although  subjected  to  a  rather  low  temperature  of  57° — 59°  F., 
the  stem  of  one  described  four  small  ellipses  in  the  course  of 
12  h.  We  shall  immediately  see  that  the  main  petiole  is  like- 
wise continually  circumnutating,  as  is  each  separate  pinna  and 
each  separate  leaflet.  Therefore,  if  the  movement  of  the  apex 
of  any  one  leaflet  were  to  be  traced,  the  course  described  would 
be  compounded  of  the  movements  of  four  separate  parts. 


CHAP.  VII. 


SLEEP   OF   LEAVES. 


375 


Fig.  157. 


8'3ffu.m, 


A  filament  had  been  fixed  on  the  previous  eveijng,  longi- 
tudinally to  the  main  petiole  of  a  nearly  full-grown,  highly- 
sensitive  leaf  (four  inches  in  length),  the  stem  having  been 
secured  to  a  stick  at  its  base ;  and  a  tracing  was  made  on  a 
vertical  glass  in  the  hot-house  under  a  high  temperature.  In 
the  figure  given  (Fig.  157),  the 
first  dot  was  made  at  8.30  A.M. 
August  2nd,  and  the  last  at  7 
P.M.  on  the  3rd.  During  12  h.  on 
the  first  day  the  petiole  moved 
thrice  downwards  and  twice 
upwards.  Within  the  same 
length  of  time  on  the  second 
day,  it  moved  five  times  down- 
wards and  four  times  upwards. 
As  the  ascending  and  descend- 
ing lines  do  not  coincide,  the 
petiole  manifestly  circumnu- 
tates;  the  great  evening  fall 
and  nocturnal  rise  being  an 
exaggeration  of  one  of  the  cir- 
cumnutations.  It  should,  how- 
ever, be  observed  that  the  pe- 
tiole fell  much  lower  down  in 
the  evenings  than  could  be 
seen  on  the  vertical  glass  or  is 
represented  in  the  diagram. 
After  7  P.M.  on  the  3rd  (when 
the  last  dot  in  Fig.  157  was 
made)  the  pot  was  carried  into 
a  bed-room,  and  the  petiole  was 
found  at  12.50  A.M.  (i.e.  after 
midnight)  standing  almost  up- 
right, and  much  more  highly 
inclined  than  it  was  at  10.40 
P.M.  When  observed  again  at 
4  A.M.  it  had  begun  to  fall,  and 
continued  falling  till  6.15  A.M., 
after  which  hour  it  zigzagged  and  again  circumnutated.  Similar 
observations  were  made  on  another  petiole,  with  nearly  the 
same  result. 

On  two  other  occasions  the  movement  of  the  main  petiole 
17 


Mimosa  pudica :  circunmutation  and 
nyctitropic  movement  of  main  pe- 
tiole, traced  during  34  h.  30  m. 


376  MODIFIED  CIRCUMNUTATION.  CHAP.  VII. 

was  observed  every  two  or  three  minutes,  the  plants  being  kept 
at  a  rather  high  temperature,  viz.,  on  the  first  occasion  at 
77°— 81°  F.,  and  the  filament  then  described  2|  ellipses  in  69  m. 
On  the  second  occasion,  when  the  temperature  was  81° — 86°  F., 
it  made  rather  more  than  3  ellipses  in  67  m.  Therefore, 
Fig.  157,  though  now  sufficiently  complex,  would  have  been  in- 
comparably more  so,  if  dots  had  been  made  on  the  glass  every 
2  or  3  minutes,  instead  of  every  hour  or  half-hour.  Although 
the  main  petiole  is  continually  and  rapidly  describing  small 
ellipses  during  the  day,  yet  after  the  great  nocturnal  rising 
movement  has  commenced,  if  dots  are  made  every  2  or  3 
minutes,  as  was  done  for  an  hour  between  9.30  and  10.30  P.M. 
(temp.  84°  F.),  and  the  dots  are  then  joined,  an  almost  abso- 
lutely straight  line  is  the  result. 

To  show  that  the  movement  of  the  petiole  is  in  all  proba- 
bility due  to  the  varying  turgescence  of  the  pulvinus,  and  not 
to  growth  (in  accordance  with  the  conclusions  of  Pfeffer),a  very 
old  leaf,  with  some  of  its  leaflets  yellowish  and  hardly  at  all 
sensitive,  was  selected  for  observation,  and  the  plant  was  kept 
at  the  highly  favourable  temp,  of  80°  F.  The  petiole  fell  from 
8  A.M.  till  10.15  A.M.,  it  then  rose  a  little  in  a  somewhat  zigzag 
line,  often  remaining  stationary,  till  5  P.M.,  when  the  great 
evening  fall  commenced,  which  was  dmtinued  till  at  least 
10  P.M.  By  7  A.M.  on  the  following  morning  it  had  risen  to  the 
same  level  as  on  the  previous  morning,  and  then  descended  in 
a  zigzag  line.  But  from  10.30  A.M.  till  4.15  P.M.  it  remained 
almost  motionless,  all  power  of  movement  being  now  lost.  The 
petiole,  therefore,  of  this  very  old  leaf,  which  must  have  long 
ceased  growing,  moved  periodically ;  but  instead  of  circum- 
nutating  several  times  during  the  day,  it  moved  ouly  twice 
down  and  twice  up  in  the  course  of  24  h.,  with  the  ascending 
and  descending  lines  not  coincident. 

It  has  already  been  stated  that  the  pinnse  move  independently 
of  the  main  petiole.  The  petiole  of  a  leaf  was  fixed  to  a  cork 
support,  close  to  the  point  whence  the  four  pinnse  diverge,  with 
a  short  fine  filament  cemented  longitudinally  to  one  of  the  two 
terminal  pinnae,  and  a  graduated  semicircle  was  placed  close 
beneath  it.  By  looking  vertically  down,  its  angular  or  lateral 
movements  could  be  measured  with  accuracy.  Between  noon 
and  4.15  r  M.  the  pinna  changed  its  position  to  one  side  by  only 
7°;  but  not  continuously  in  the  same  direction,  as  it  moved 
four  times  to  one  side,  and  three  times  to  the  opposite  side, 


CHAP.  VII.  SLEEP  OF  LEAVES.  377 

in  one  instance  to  the  extent  of  16°.  This  pinna,  therefore, 
circumnutated.  Later  in  the  evening  the  four  pinnae  approach 
each  other,  and  the  one  which  was  observed  moved  inwards 
59°  between  noon  and  6.45  P.M.  Ten  observations  were  made 
in  the  course  of  2  h.  20  m.  (at  average  intervals  of  14  m.), 
between  4.25  and  6.45  P.M.  ;  and  there  was  now,  when  the  leaf 
was  going  to  sleep,  no  swaying  from  side  to  side,  but  a  steady 
inward  movement.  Here  therefore  there  is  in  the  evening  the 
same  conversion  of  a  circumnutating  into  a  steady  movement 
in  one  direction,  as  in  the  case  of  the  main  petiole. 

It  has  also  been  stated  that  each  separate  leaflet  circum- 
nutates.  A  pinna  was  cemented  with  shellac  on  the  summit  of 
a  little  stick  driven  firmly  into  the  ground,  immediately  beneath 
a  pair  of  leaflets,  to  the  midribs  of  both  of  which  excessively 
fine  glass  filaments  were  attached.  This  treatment  did  not 
injure  the  leaflets,  for  they  went  to  sleep  in  the  usual  manner, 
and  long  retained  their  sensitiveness.  The  movements  of  one 
of  them  were  traced  during  49  h.,  as  shown  in  Fig.  158.  On  the 
first  day  the  leaflet  sank  down  till  11.30  A.M.,  and  then  rose 
till  late  in  the  evening  in  a  zigzag  line,  indicating  circum- 
nutation.  On  the  second  day,  when  more  accustomed  to  its 
new  state,  it  oscillated  twice  up  and  twice  down  during  the 
24  h.  This  plant  w*  subjected  to  a  rather  low  temperature, 
viz.,  62° — 64°  F. ;  had  it  been  kept  warmer,  no  doubt  the  move- 
ments of  the  leaflet  would  have  been  much  more  rapid  and 
complicated.  It  may  be  seen  in  the  diagram  that  the  ascending 
and  descending  lines  do  not  coincide ;  but  the  large  amount  of 
lateral  movement  in  the  evening  is  the  result  of  the  leaflets 
bending  towards  the  apex  of  the  leaf  when  going  to  sleep. 
Another  leaflet  was  casually  observed,  and  found  to  be  con- 
tinually circumnutating  during  the  same  length  of  time. 

The  circumnutation  of  the  leaves  is  not  destroyed  by  their 
being  subjected  to  moderately  long  continued  darkaess ;  but  the 
proper  periodicity  of  their  movements  is  lost.  Some  very  young 
seedlings  were  kept  during  two  days  in  the  dark  (temp.  57° — 59° 
F.),  except  when  the  circumnutatiou  of  their  stems  was  occa- 
sionally observed ;  and  on  the  evening  of  the  second  day  the 
leaflets  did  not  fully  and  properly  go  to  sleep.  The  pot  was 
then  placed  for  three  days  in  a  dark  cupboard,  under  nearly  the 
same  temperature,  and  at  the  close  of  this  period  the  leaflets 
showed  no  signs  of  sleeping,  and  were  only  slightly  sensitive  to 
a  touch.  On  the  following  day  the  stem  was  cemented  to  a 


378 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VII. 


stick,  and  the  movements  of  two  leaves  were  traced  on  a  vertical 
glass  during  72  h.  The  plants  were  still  kept  in  the  dark,  ex- 
cepting that  at  each  observation,  which  lasted  3  or  4  minutes, 

Fig.  158. 


Mimosa  pudica:  circumnutation  and  nyctitropic  movement  of  a  leaflet 
(with  pinna  secured),  traced  on  a  vertical  glass,  from  8  A.M.  Sept.  14th 
to  9  A.M.  16th. 

they  were  illuminated  by  two  candles.  On  the  third  day  the 
leaflets  still  exhibited  a  vestige  of  sensitiveness  when  forcibly 
pressed,  but  in  the  evening  they  showed  no  signs  of  sleep. 
Nevertheless,  their  petioles  continued  to  circumnutate  distinctly, 


CHAP.  VII. 


SLEEP  OF  LEAVES. 


379 


although  the  proper  order  of  their  movements  in  relation  to  the 
day  and  night  was  wholly  lost.  Thus,  one  leaf  descended  during 
the  first  two  nights  (i.e.  between  10  P.M.  and  7  A.M.  next  morn- 
ing) instead  of  ascending,  and  on  the  third  night  it  moved 
chiefly  in  a  lateral  direction.  The  second  leaf  behaved  in  an 
equally  abnormal  manner,  moving  laterally  during  the  first 
night,  descending  greatly  during  the  second,  and  ascending  to 
an  unusual  height  during  the  third  night. 

With  plants  kept  at  a  high  temperature  and  exposed  to  the 
light,  the  most  rapid  circumnutating  movement  of  the  apex 
of  a  leaf  which  was  observed,  amounted  to  ^^  of  an  inch  in 
one  second;  and  this  would  have  equalled  £  of  an  inch  in  a 
minute,  had  not  the  leaf  occasionally  stood  still.  The  actual 
distance  travelled  by  the  apex  (as  ascertained  by  a  measure 
placed  close  to  the  leaf)  was  on  one  occasion  nearly  f  of  an  inch 
in  a  vertical  direction  in  15  m. ;  and  on  another  occasion  -g-  of  an 
inch  in  60  m. ;  but  there  was  also  some  lateral  movement. 

Mimosa  albida* — The  leaves  of  this  plant,  one  of  which  is  here 
figured  (Fig.  159)  reduced  to  f  of  the  natural  size,  present  some 

Fig.  159. 


Mimosa  albida  :  leaf  seen  from  vertically  above. 

interesting  peculiarities.  It  consists  of  a  long  petiole  bearing 
only  two  pinnae  (here  represented  as  rather  more  divergent 
than  is  usual),  each  with  two  pairs  of  leaflets.  But  the  inner 


*  Mr.  Thistleton  Dyer  informs 
us  that  this  Peruvian  plant  (which 
was  sent  to  us  from  Kew)  is  con- 
sidered by  Mr.  Bentham  ('  Trans. 


Linn.  Soc.,'  vol.  xxx.  p.  390)  to 
be  "  the  species  or  variety  which 
most  commonly  represents  the  M. 
sensitiva  of  our  gardens." 


380  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

basal  leaflets  are  greatly  reduced  in  size,  owing  probably  to  the 
want  of  space  for  their  full  development,  so  that  they  may  be 
considered  as  almost  rudimentary.  They  vary  somewhat  in 
size,  and  both  occasionally  disappear,  or  only  one.  Neverthe- 
less, they  are  not  in  the  least  rudimentary  in  function,  for  they 
are  sensitive,  extremely  heliotropic,  circumnutate  at  nearly  the 
same  rate  as  the  fully  developed  leaflets,  and  assume  when 
asleep  exactly  the  same  position.  With  M.  pudica  the  inner 
leaflets  at  the  base  and  between  the  pinna)  are  likewise  much 
shortened  and  obliquely  truncated ;  this  fact  was  well  seen  in 
some  seedlings  of  M.  pudica,  in  which  the  third  leaf  above  the 
cotyledons  bore  only  two  pinnae,  each  with  only  3  or  4  pairs  of 
leaflets,  of  which  the  inner  basal  one  was  less  than  half  as  long 
as  its  fellow;  so  that  the  whole  leaf  resembled  pretty  closely 
that  of  M.  alUda.  In  this  latter  species  the  main  petiole  termi- 
nates in  a  little  point,  and  on  each  side  of  this  there  is  a  pair 
of  minute,  flattened,  lancet- shaped  projections,  hairy  on  their 
margins,  which  drop  off  and  disappear  soon  after  the  leaf  is 
fully  developed.  There  can  hardly  be  a  doubt  that  these  little 
projections  are  the  last  and  fugacious  representatives  of  an 
additional  pair  of  leaflets  to  each  pinna;  for  the  outer  one  is 
twice  as  broad  as  the  inner  one,  and  a  little  longer,  viz.  T£^  of  an 
inch,  whilst  the  inner  one  is  only  |^§  long.  Now  if  the  basal 
pair  of  leaflets  of  the  existing  leaves  were  to  become  rudimen- 
tary, we  should  expect  that  the  rudiments  would  still  exhibit 
some  trace  of  their  present  great  inequality  of  size.  The  con- 
clusion that  the  pinnae  of  the  parent-form  of  M.'alUda  possessed 
at  least  three  pairs  of  leaflets,  instead  of,  as  at  present,  only  two, 
is  supported  by  the  structure  of  the  first  true  leaf;  for  this 
consists  of  a  simple  petiole,  often  bearing  three  pairs  of  leaflets. 
This  latter  fact,  as  well  as  the  presence  of  the  rudiments,  both 
lead  to  the  conclusion  that  M.  alUda  is  descended  from  a  form 
the  leaves  of  which  bore  more  than  two  pairs  of  leaflets.  The 
second  leaf  above  the  cotyledons  resembles  in  all  respects  the 
leaves  on  fully  developed  plants. 

When  the  leaves  go  to  sleep,  each  leaflet  twists  half  round, 
so  as  to  present  its  edge  to  the  zenith,  and  comes  into  close 
contact  with  its  fellow.  The  pinnse  also  approach  each  other 
closely,  so  that  the  four  terminal  leaflets  come  together.  The 
large  basal  leaflets  (with  the  little  rudimentary  ones  in  contact 
with  them)  move  inwards  and  forwards,  so  as  to  embrace  the 
outside  of  the  united  terminal  leaflets,  and  thus  all  eight  leaflets 


CHAP.  VII.  SLEEP  OF   LEAVES:  381 

(the  rudimentary  ones  included)  form  together  a  single  vertical 
packet.  The  two  pinnae  at  the  same  time  that  they  approach 
each  other  sink  downwards,  and  thus  instead  of  extending  hori- 
zontally in  the  same  line  with  the  main  petiole,  as  during  the 
day,  they  depend  at  night  at  about  45°,  or  even  at  a  greater 
angle,  beneath  the  horizon.  The  movement  of  the  main  petiole 
seems  to  be  variable ;  we  have  seen  it  in  the  evening  27°  lower 
than  during  the  day ;  but  sometimes  in  nearly  the  same  position. 
Nevertheless,  a  sinking  movement  in  the  evening  and  a  rising- 
one  during  the  night  is  probably  the  normal  course,  for  this 
was  well-marked  in  the  petiole  of  the  first-formed  true  leaf. 

The  circumnutation  of  the  main  petiole  of  a  young  leaf  was 
traced  during  21  days,  and  was  considerable  in  extent,  but  less 
complex  than  that  of  M.  pudica.  The  movement  was  much 
more  lateral  than  is  usual  with  circumnutating  leaves,  and  this 
was  the  sole  peculiarity  which  it  presented.  The  apex  of 
one  of  the  terminal  leaflets  was  seen  under  the  microscope  to 
travel  ^  of  an  inch  in  3  minutes. 

Mimosa  marginata. — The  opposite  leaflets  rise  up  and  approach 
each  other  at  night,  but  do  not  come  into  close  contact,  except  in 
the  case  of  very  young  leaflets  on  vigorous  shoots.  Full-grown 
leaflets  circumnutate  during  the  day  slowly  and  on  a  small  scale. 

Schrankia  uncinata  (Tribe  20). — A  leaf  consists  of  two  or  three 
pairs  of  pinnae,  each  bearing  many  small  leaflets.  These,  when 
the  plant  is  asleep,  are  directed  forwards  and  become  imbricated. 
The  angle  between  the  two  terminal  pinnae  was  diminished  at 
night,  in  one  case  by  15° ;  and  they  sank  almost  vertically  down- 
wards. The  hinder  pairs  of  pinnae  likewise  sink  downwards, 
but  do  not  converge,  that  is,  move  towards  the  apex  of  the  leaf. 
The  main  petiole  does  not  become  depressed,  at  least  during  the 
evening.  In  this  latter  respect,  as  well  as  in  the  sinking  of  the 
pinnae,  there  is  a  marked  difference  between  the  nyctitropic 
movements  of  the  present  plant  and  of  Mimosa  pudica.  It 
should,  however,  be  added  that  our  specimen  was  not  in  a  very 
vigorous  condition.  The  pinnae  of  Schrankia  acuhata  also  sink 
at  night. 

Acacia  Farnesiana  (Tribe  22). — The  different  appearance  pre- 
sented by  a  bush  of  this  plant  when  asleep  and  awake  is  won- 
derful. The  same  leaf  in  the  two  states  is  shown  in  the  following 
figure  (Fig.  160).  The  leaflets  move  towards  the  apex  of  the 
pinna  and  become  imbricated,  and  the  pinnae  then  look  like  bits 
of  dangling  string.  The  following  remarks  and  measurements 


382 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VII. 


do  not  fully  apply  to  the  small  leaf  here  figured.  The  pinme 
move  forwards  and  at  the  same  time  sink  downwards,  whilst 
the  main  petiole  rises  considerably.  With  respect  to  the  degree 
of  movement :  the  two  terminal  pinnae  of  one  specimen  formed 
together  an  angle  of  100°  during  the  day,  and  at  night, of  only 
38°,  so  each  had  moved  31°  forwards.  The  penultimate  pinnae 
during  the  day  formed  together  an  angle  of  180°,  that  is,  they 
stood  in  a  straight  line  opposite  one  another,  and  at  night  each 
had  moved  65°  forwards.  The  basal  pair  of  pinnse  were  directed 

Fig.  160. 


A.  B. 

Acacia  Farnesiana;  A,  leaf  during  the  day;  B,  the  same  leaf  at  night. 

during  the  day,  each  about  21°  backwards,  and  at  night  38° 
forwards,  so  each  had  moved  59°  forwards.  But  the  pinnse  at 
the  same  time  sink  greatly,  and  sometimes  hang  almost  perpen- 
dicularly downwards.  The  main  petiole,  on  the  other  hand, 
rises  much :  by  8.30  P.M.  one  stood  34°  higher  than  at  noon, 
and  by  6.40  A.M.  on  the  following  morning  it  was  still  higher 
by  10°;  shortly  after  this  hour  the  diurnal  sinking  move- 
ment commenced.  The  course  of  a  nearly  full-grown  leaf  was 
traced  during  14  h. ;  it  was  strongly  zigzag,  and  apparently 


CHAP.  VII.  SLEE1P   OF  LEAVES.  383 

represented  five  ellipses,  with  their  longer  axes  differently 
directed. 

Albizzia  lophantha  (Tribe  23). — The  leaflets  at  night  come  into 
contact  with  one  another,  and  are  directed  towards  the  apex  of 
the  pinna.  The  pinnae  approach  one  another,  but  remain  in  the 
same  plane  as  during  the  day ;  and  in  this  respect  they  differ 
much  from  those  of  the  above  Schrankia  and  Acacia.  The  main 
petiole  rises  but  little.  The  first-formed  leaf  above  the  coty- 
ledons bore  11  leaflets  on  each  side,  and  these  slept  like  those 
on  the  subsequently  formed  leaves ;  but  the  petiole  of  this  first 
leaf  was  curved  downwards  during  the  day  and  at  night 
straightened  itself,  so  that  the  chord  of  its  arc  then  stood  16° 
higher  than  in  the  day-time. 

Melahuca  ericcefolia  (Myrtaceae). — According  to  Bouche  ('  Bot. 
Zeit.,'  1874,  p.  359)  the  leaves  sleep  at  night,  in  nearly  the  same 
manner  as  those  of  certain  species  of  Pimelia. 

(Enothera  mollissima  (Onagrariese). — According  to  Linnseus 
('  Somnus  Plantarum '),  the  leaves  rise  tip  vertically  at  night. 

Passiflora  gracilis  (Passifloracse). — The  young  leaves  sleep  by 
their  blades  hanging  vertically  downwards,  and  the  whole  length 
of  the  petiole  then  becomes  somewhat  curved  downwards. 
Externally  no  trace  of  a  pulvinus  can  be  seen.  The  petiole  of 
the  uppermost  leaf  on  a  young  shoot  stood  at  10.45  A.M.  at  33° 
above  the  horizon ;  and  at  10.30  P.M.,  when  the  blade  was  verti- 
cally dependent,  at  only  15°,  so  the  petiole  had  fallen  18°.  That 
of  the  next  older  leaf  fell  only  7°.  From  some  unknown  cause 
the  leaves  do  not  always  sleep  properly.  The  stem  of  a  plant, 
which  had  stood  for  some  time  before  a  north-east  window,  was 
secured  to  a  stick  at  the  base  of  a  young  leaf,  the  blade  of 
which  was  inclined  at  40°  below  the  horizon.  From  its  position 
the  leaf  had  to  be  viewed  obliquely,  consequently  the  vertically 
ascending  and  descending  movements  appeared  when  traced 
oblique.  On  the  first  day  (Oct.  12th)  the  leaf  descended  in  a 
zigzag  line  until  late  in  the  evening;  and  by  8.15  A.M.  on  the 
13th  had  risen  to  nearly  the  same  level  as  on  the  previous 
morning.  A  new  tracing  was  now  begun  (Fig.  161).  The 
leaf  continued  to  rise  until  8.50  A.M.,  then  moved  a  little  to  the 
right,  and  afterwards  descended.  Between  11  A.M.  and  5  P.M.  it 
circumnutated,  and  after  the  latter  hour  the  great  nocturnal 
fall  commenced.  At  7.15  P.M.  it  depended  vertically.  The 
dotted  line  ought  to  have  been  prolonged  much  lower  down  in 
the  figure.  By  6.50  A.M.  on  the  following  morning  (14th)  the 


381 


MODIFIED   CIKCUMNUTATION. 


CHAP.  VII. 


leaf  had  risen  greatly,  and  continued  to  rise  till  7.50  A.M.,  after 
which  hour  it  redescended.  It  should  be  observed  that  the  lines 
traced  on  this  second  morning  would  have  coincided  with  and 
confused  those  previously  traced,  had  not  the  pot  been  slided 
a  very  little  to  the  left.  In  the  evening  (14th)  a  mark  was 
placed  behind  the  filament  attached  to  the  apex  of  the  leaf,  and 
its  movement  was  carefully  traced  from  5  P.M.  to  10.15  P.M. 

Fig.  161. 


Passiflora  gracilis:  circumnutation  and  nyctitropic  movement  of  leaf, 
traced  on  vertical  glass,  from  8.20  A.M.  Oct.  13th  to  10  A.M.  14th 
Figure  reduced  to  two-thirds  of  original  scale. 

Between  5  and  7.15  P.M.  the  leaf  descended  in  a  straight  line, 
and  at  the  latter  hour  it  appeared  vertically  dependent.  But 
between  7.15  and  10.15  P.M.  the  line  consisted  of  a  succession 
of  steps,  the  cause  of  which  we  could  not  understand ;  it  was, 
however,  manifest  that  the  movement  was  no  longer  a  simple 
descending  one. 

Sirgesbeckia  orientalis  (Compositse). — Some  seedlings  were 
raised  in  the  middle  of  winter  and  kept  in  the  hot-house ;  they 
flowered,  but  did  not  grow  well,  and  their  leaves  never  showed 
any  signs  of  sleep.  The  leaves  on  other  seedlings  raised  in  May 
w«re  horizontal  at  noon  (June  22nd),  and  depended  at  a  consi- 


CHAP.  VII. 


SLEEP  OF  LEAVES. 


385 


derable  angle  beneath  the  horizon  at  10  P.M.  In  the  case  of  four 
youngish  leaves,  which  were  from  2  to  2£  inches  in  length, 
these  angles  were  found  to  be  50°,  56°,  60°,  and  65°.  At  the 
end  of  August,  when  the  plants  had  grown  to  a  height  of  10  to  11 
inches,  the  younger  leaves  were  so  much  curved  downwards  at 
night  that  they  might  truly  be  said  to  be  asleep.  This  is  one 

Fig.  162. 


Nicotiana  glauca :  shoots  with  leaves  expanded  during  the  day,  and  asleep 
at  night.     Figures  copied  from  photographs,  and  reduced. 

of  the  species  which  must  be  well  illuminated  during  the  day 
in  order  to  sleep,  for  on  two  occasions  when  plants  were  kept 
all  day  in  a  room  with  north-east  windows,  the  leaves  did  not 
sleep  at  night.  The  same  cause  probably  accounts  for  the 
leaves  on  our  seedlings  raised  in  the  dead  of  the  winter  not 
sleeping.  Professor  Pfeffer  informs  us  that  the  leaves  of 
another  species  (S.  Jorullensis  ?)  hang  vertically  down  at  night. 


386 


MODIFIED  CIRCUMNUTATION. 


CHAP.  VII. 


Ijomoea  ccerulea  and  purpurea  (Convolvulaceae).  — The  leaves  on 
yery  young  plants,  a  foot  or  two  in  height,  are  depressed  at  night 

to  between  68°  and  80° 
beneath 
and 


Fig.  163. 

i\ 

ujr.*f>0u0? 


the  horizon ; 
some  hang  quite 
vertically  downwards. 
On  the  following  morn- 
ing they  again  rise  into 
a  horizontal  position. 
The  petioles  become 
at  night  downwardly 
curved,  either  through 
their  entire  length  or  in 
the  upper  part  alone ; 
and  this  apparently 
causes  the  depression 
of  the  blade.  It  seems 
necessary  that  the 
leaves  should  be  well 
illuminated  during  the 
day  in  order  to  sleep, 
for  those  which  stood 
on  the  back  of  a  plant 
before  a  north-east 
window  did  not  sleep. 

Nicotiana  tabacum 
(var.  Virginian)  and 
glauca  (Solanese).— The 
young  leaves  of  both 
these  species  sleep  by 
bendinh  vertically  up- 
wards. Figures  of  two 
shoots  of  N.  glauca, 
awake  and  asleep  (Fig. 
162),  are  given  on  p. 
385  :  one  of  the  shoots, 
from  which  the  photo- 
graphs were  taken,  was 
accidentally  bent  to  one 
side. 

At  the  base  of  the  petiole  of  N.  tabacum,  on  the  outside,  there 
is  a  mass  of  cells,  which  are  rather  smaller  than  elsewhere,  and 


Nicotiana  tabacum  :  circumnutation  and  nyc- 
titropic  movement  of  a  leaf  (5y-  inches  in 
length),  traced  on  a  vertical  glass,  from 
3  P.M.  July  10th  to  8.10  A.M.  13th.  Apex 
of  leaf  4  inches  from  glass.  Temp.  17  J°- 
18.j°  C.  Figure  reduced  to  one-half 
original  scale. 


CHAP.  VII.  SLEEP   OF  LEAVES.  387 

have  their  longer  axes  differently  directed  from  the  cells  of  the 
parenchyma,  and  may  therefore  be  considered  as  forming  a  sort 
of  pulvinus.  A  young  plant  of  N.  tabacum  was  selected,  and 
the  circumnutation  of  the  fifth  leaf  above  the  cotyledons  was 
observed  during  three  days.  On  the  first  morning  (July  10th) 
the  leaf  fell  from  9  to  10  A.M.,  which  is  its  normal  course,  but 
rose  during  the  remainder  of  the  day ;  and  this  no  doubt  was 
due  to  its  being  illuminated  exclusively  from  above;  for  properly 
the  evening  rise  does  not  commence  until  3  or  4  P.M.  In  the 
figure  as  given  on  p.  386  (Fig.  163)  the  first  dot  was  made  at 
3  P.M.  ;  and  the  tracing  was  continued  for  the  following  65  h. 
When  the  leaf  pointed  to  the  dot  next  above  that  marked  3  P.M., 
it  stood  horizontally.  The  tracing  is  remarkable  only  from  its 
simplicity  and  the  straightness  of  the  lines.  The  leaf  each  day 
described  a  single  great  ellipse ;  for  it  should  be  observed  that 
the  ascending  and  descending  lines  do  not  coincide.  On  the 
evening  of  the  llth  the  leaf  did  not  descend  quite  so  low  as 
usual,  and  it  now  zigzagged  a  little.  The  diurnal  sinking  move- 
ment had  already  commenced  each  morning  by  7  A.M.  The  broken 
lines  at  the  top  of  the  figure,  representing  the  nocturnal  vertical 
position  of  the  leaf,  ought  to  be  prolonged  much  higher  up. 

Mirabitts  longiflora  and  jalapa  (Nyctagineae). — The  first  pair 
of  leaves  above  the  cotyledons,  produced  by  seedlings  of  both 
these  species,  were  considerably  divergent  during  the  day,  and 
at  night  stood  up  vertically  in  close  contact  with  one  another. 
The  two  upper  leaves  on  an  older  seedling  were  almost  horizontal 
by  day,  and  at  night  stood  up  vertically,  but  were  not  in  close 
contact,  owing  to  the  resistance  offered  by  the  central  bud. 

Polygonum  aviculare  (Polygonese).— Professor  Batalin  informs 
us  that  the  young  leaves  rise  up  vertically  at  night.  This  is 
likewise  the  case,  according  to  Linnaeus,  with  several  species 
of  Amaranthus  (Amaranthacese) ;  and  we  observed  a  sleep  move- 
ment of  this  kind  in  one  member  of  the  genus.  Again,  with 
Chenopodium  album  (Chenopodieae),  the  upper  young  leaves  of, 
some  seedlings,  about  4  inches  in  height,  were  horizontal  or 
sub-horizontal  during  the  day,  and  at  10  P.M.  on  March  7th 
were  quite,  or  almost  quite,  vertical.  Other  seedlings  raised  in 
the  greenhouse  during  the  winter  (Jan.  28th)  were  observed  day 
and  night,  and  no  difference  could  be  perceived  in  the  position 
of  their  leaves.  According  to  Bouche  ('  Bot.  Zeitung/  1874, 
p.  359;  the  leaves  of  Pimdia,  linoides  and  spectabilis  (Thymelese) 
sleep  at  night. 


388  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

Euphorbia  jacguitiiceflora  (Euphorbiacese).  —  Mr.  Lynch 
called  our  attention  to  the  fact  that  the  young  leaves  of  this 
plant  sleep  by  depending  vertically.  The  third  leaf  from  the 
summit  (March  IJth)  was  inclined  during  the  day  30°  beneath 
the  horizon,  and  at  night  hung  vertically  down,  as  did  some  of 
the  still  younger  leaves.  It  rose  up  to  its  former  level  on  the 
following  morning.  The  fourth  and  fifth  leaves  from  the  summit 
stood  horizontally  during  the  day,  and  sank  down  at  night  only 
38°.  The  sixth  leaf  did  not  sensibly  alter  its  position.  The 
sinking  movement  is  due  to  the  downward  curvature  of  the 
petiole,  no  part  of  which  exhibits  any  structure  like  that  of 
a  pulvinus.  Early  on  the  morning  of  June  7th  a  filament  was 
fixed  longitudinally  to  a  young  leaf  (the  third  from  the  summit, 
and  2f  inches  in  length),  and  its  movements  were  traced  on 
a  vertical  glass  during  72  h.,  the  plant  being  illuminated  from 
above  through  a  skylight.  Each  day  the  leaf  fell  in  a  nearly 
straight  line  from  7  A.M.  to  5  P.M.,  after  which  hour  it  was  so 
much  inclined  downwards  that  the  movement  could  no  longer 
be  traced ;  and  during  the  latter  part  of  each  night,  or  early  in 
the  morning,  the  leaf  rose.  It  therefore  circumnutated  in  a 
very  simple  manner,  making  a  single  large  ellipse  every  24  h., 
for  the  ascending  and  descending  lines  did  not  coincide.  On 
each  successive  morning  it  stood  at  a  less  height  than  on  the 
previous  one,  and  this  was  probably  due,  partly  to  the  increasing 
age  of  the  leaf,  and  partly  to  the  illumination  being  insufficient ; 
for  although  the  leaves  are  very  slightly  heliotropic,  yet,  accord- 
ing to  Mr.  Lynch's  and  our  own  observations,  their  inclination 
during  the  day  is  determined  by  the  intensity  of  the  light.  On 
the  third  day,  by  which  time  the  extent  of  the  descending 
movement  had  much  decreased,  the  line  traced  was  plainly 
much  more  zigzag  than  on  any  previous  day,  and  it  appeared 
as  if  some  of  its  powers  of  movement  were  thus  expended.  At 
10  P.M.  on  June  7th,  when  the  leaf  depended  vertically,  its  move- 
ments were  observed  by  a  mark  being  placed  behind  it,  and  the 
end  of  the  attached  filament  was  seen  to  oscillate  slowly  and 
slightly  from  side  to  side,  as  well  as  upwards  and  downwards. 

Phyllanthus  Niruri  (Euphorbiacese).  —  The  leaflets  of  this 
plant  sleep,  as  described  by  Pfeffer,*  in  a  remarkable  manner, 
apparently  like  those  of  Cassia,  for  they  sink  downwards  at 
night  and  twist  round,  so  that  their  lower  surfaces  are  turned 

*  « Die  Period.  Beweg.,'  p.  159. 


CHAP.  VII.  SLEEP  OF  LEAVES  389 

outwards.    They  are  furnished,  .as  might  have  been  expected 
from  tliis  complex  kind  of  movement,  with  a  pulvinus.  * 

GYMNOSPERMS. 

Pinus  Nordmanniana  (Coniferae). — M.  Chatin  states  *  that  the 
leaves,  which  are  horizontal  during  the  day/  rise  up  at  night,  so 
a-i  to  assume  a  position  almost  perpendicular  to  the  branch  from 
which  they  arise ;  we  presume  that  he  here  refers  to  a  horizontal 
branch.  He  adds  :  "  En  meme  temps,  ce  mouvement  direction 
est  accompagne  d'un  mouvement  de  torsion  imprime  a  la  partie 
basilaire  de  la  feuille,  et  pouvant  sou  vent  parcourir  un  arc  de 
90  degres."  As  the  lower  surfaces  of  the  leaves  are  white, 
whilst  the  upper  are  dark  green,  the  tree  presents  a  widely 
different  appearance  by  day  and  night.  The  leaves  on  a  small 
tree  in  a  pot  did  not  exhibit  with  us  any  nyctitropic  move- 
ments. We  have  seen  in  a  former  chapter  that  the  leaves  of 
Pinus  pinaster  and  Austriuca  are  continually  circumnutating. 


MONOCOTYLEDONS. 

Thalia  dealbata  (Cannacese). — The  leaves  of  this  plant  sleep 
by  turning  vertically  upwards ;  they  are  furnished  with  a  well- 
developed  pulvinus.  It  is  the  only  instance  known  to  us  of 
a  very  large  leaf  sleeping.  The  blade  of  a  young  leaf,  which 
was  as  yet  only  131  inches  in  length  and  62  in  breadth,  formed 
at  noon  an  angle  with  its  tall  petiole  of  121°,  and  at  night  stood 
vertically  in  a  line  with  it,  and  so  had  risen  59°.  The  actual 
distance  travelled  by  the  apex  (as  measured  by  an  orthogonic 
tracing)  of  another  large  leaf,  between  7.30  A.M.  and  10  P.M.,  was 
10i  inches.  The  circumnutation  of  two  young  and  dwarfed 
leaves,  arising  amongst  the  taller  leaves  at  the  base  of  the  plant, 
was  traced  on  a  vertical  glass  during  two  days.  On  the  first  day 
the  apex  of  one,  and  on  the  second  day  the  apex  of  the  other  leaf, 
described  between  6.40  A.M.  and  4  p  M.  two  ellipses,  the  longer 
axes  of  which  were  extended  in  very  different  directions  from  the 
lines  representing  the  great  diurnal  sinking  and  nocturnal  rising 
movement. 

Maranta  arundinacea  (Cannacese). — The  blades  of  the  leaves, 
which  are  furnished  with  a  pulvinus,  stand  horizontally  during 


*  '  Comptes  Rendus,'  -Jan.  1876,  p.  171. 


390  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

the  day  or  between  10°  and  20°  above  the  horizon,  and  at  night 
vertically  upwards.  They  therefore  rise  between  70°  and  90°  at 
eight.  The  plant  was  placed  at  noon  in  the  dark  in  the  hot- 
house, and  on  the  following  day  the  movements  of  the  leaves 
were  traced.  Between  8.40  and  10.30  A.M.  they  rose,  and  then 
fell  yreatly  till  1.37  P.M.  But  by  3  P.M.  they  had  again  risen  a 
little,  and  continued  to  rise  during  the  rest  of  the  afternoon  and 
night ;  on  the  following  morning  they  stood  at  the  same  level  as 
on  the  previous  day.  Darkness,  therefore,  during  a  day  and  a 
half  does  not  interfere  with  the  periodicity  of  their  movements. 
On  a  warm  but  stormy  evening,  the  plant  whilst  being  brought 
into  the  house,  had  its  leaves  violently  shaken,  and  at  night  not 
one  went  to  sleep.  On.  the  next  morning  the  plant  was  taken 
back  to  the  hot-house,  and  again  at  night  the  leaves  did  not 
sleep ;  but  on  the  ensuing  night  they  rose  in  the  usual  manner 
between  70°  and  80°.  This  fact  is  analogous  with  what  we 
have  observed  with  climbing  plants,  namely,  that  much  agitation 
checks  for  a  time  their  power  of  circumnutation  ;  but  the  eifect 
in  this  instance  was  much  more  strongly  marked  and  prolonged. 
Colocasia  antiquorum  (Caladium  esculentum,  Hort.)  (Aroideae). 
— The  leaves  of  this  plant  sleep  by  their  blades  sinking  in  the 
evening,  so  as  to  stand  highly  inclined,  or  even  quite  vertically 
with  their  tips  pointing  to  the  ground.  They  are  not  provided 
with  a  pulvinus.  The  blade  of  one  stood  at  noon  1°  beneath  the 
horizon;  at  4.20  P.M.,  '20° ;  at  6  P.M.,  43° ;  at  7.20  P.M.,  69° ;  and  at 
8.30  P.M.,  68° ;  so  it  had  now  begun  to  rise  ;  at  10.15  P.M.  it  stood 
at  65°,  and  on  the  following  early  morning  at  11°  beneath  the 
horizon.  The  circumnutation  of  another  young  leaf  (with  its 
petiole  only  3}  inches,  and  the  blade  4  inches  in  length),  was 
traced  on  a  vertical  glass  during  48  h. ;  it  was  dimly  illuminated 
through  a  skylight,  and  this  seemed  to  disturb  the  proper  perio- 
dicity of  its  movements.  Nevertheless,  the  leaf  fell  greatly 
during  both  afternoons,  till  either  7.10  PM.  or  9  P.M.,  when  it 
rose  a  little  and  moved  laterally.  By  an  early  hour  on  both 
mornings,  it  had  assumed  its  diurnal  position.  The  well-marked 
lateral  movement  for  a  short  time  in  the  early  part  of  the  night, 
was  the  only  interesting  fact  which  it  presented,  as  this  caused 
the  ascending  and  descending  lines  not  to  coincide,  in  accord- 
ance with  the  general  rule  with  circumnutating  organs.  The 
movements  of  the  leaves  of  this  plant  are  thus  of  the  most 
simple  kind;  and  the  tracing  is  not  -worth  giving.  We  have 
seen  that  in  another  genus  of  the  Aroideae,  namely,  Pistia,  the 


CHAP.  VII.  SLEEP   OF   LEAVES.  391 

leaves  rise  so  much  at  night  that  they  may  almost  be  said  to 
sleep. 

titrephium  floribundum*  (Graminese).  —  The  oval  leaves  are 
provided  with  a  pulvinus,  and  are  extended  horizontally  or 
declined  a  little  beneath  the  horizon  during  the  day.  Those 
on  the  upright  culms  simply  rise  up  vertically  at  night,  so 
that  their  tips  are  directed  towards  the  zenith.  (Fig.  164.) 

Fig   164. 


Strephiurn  floribundum :  culms  with  leaves  during  the  day,  and  when  asleep 
at  night.     Figures  reduced. 

Horizontally  extended  leaves  arising  from  much  inclined  or 
almost  horizontal  culms,  move  at  night  so  that  their  tips 
point  towards  the  apex  of  the  culm,  with  one  lateral  margin 
directed  towards  the  zenith;  and  in  order  to  assume  this 
position  the  leaves  have  to  twist  on  their  own  axes  through  an 
angle  of  nearly  90°.  Thus  the  surface  of  the  blade  always  stands 
vertically,  whatever  may  be  the  position  of  the  midrib  or  of  the 
leaf  as  a  whole. 

The  circumnutation  of  a  young  leaf  (2  •  3  inches  in  length)  was 
traced  during  48  h.  (Fig.  165).  The  movement  was  remarkably 
simple;  the  leaf  descended  from  before  6.40  A.M.  until  2  or 
2.50  P.M.,  and  then  rose  so  as  to  stand  vertically  at  about  6  P.M., 
descending  again  late  in  the  night  or  in  the  very  early  morning. 


*  A.  Brongniart  first  observed      la  Soc.  Bot.  de  France,'  torn.  vii. 
that  the  leaves  of  this  plant  and       1860,  p.  470. 
of  Marsilea  sleep :  see  '  Bull,  de 


392 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VII. 


On  the  second  day  the  descending  line  zigzagged  slightly.     As 
picr  165  usual,  the   ascending   and    de- 

"  scending  lines  did  not  coincide. 

J  On  another  occasion,  when  the 

i  temperature  was  a  little  higher, 

/  viz.,   24°-26£°  C.,    a  leaf   was 

I  observed  17  times  between  8.50 

A.M.  and  12.16  P.M.  ;  it  changed 
£  its   course  by  as    much    as   a 

I  rectangle  six  times  in  this  in- 

/  terval  of  3  h.  26  m.,  and  de- 

/  .scribed  two  irregular  triangles 

/  and  a  half.     The  leaf,  therefore, 

!  on  this  occasion  circumnutated 

japidly     and     in     a     complex 
manner. 


ACOTYLEDONS. 

Marsilea    quadiifoliata    (Mar- 
sileacese). — The  shape  of  a  leaf, 
expanded    horizontally   during 
the  day,  is  shown  at  A  (Fig.  166). 
Each  leaflet  is   provided  with 
a     well-developed       pulvinus. 
"When  the  leaves  sleep,  the  two 
terminal  leaflets  rise   up,  twist 
half  round  and  come  into  con- 
tact with  one  another  (B),  and 
are  afterwards  embraced  by  the 
two  lower  leaflets  (C);  so  that 
the  four  leaflets  with  their  lower 
surfaces  turned  outwards  form 
a  vertical  packet.     The  curva- 
ture of  the  summit  of  the  petiole 
of  the    leaf    figured  asleep,  is 
Strephium  floribundwn :  circumnu-     merely  accidental.     The    plant 
tation  and  nyctitropic  movement      was  brought  into  a  room,  where 
&ttt^&V*±t     *e  temperature  was  on]y  a  little 
fixed  along  the  midrib.    Apex  of     above  60°  F.,  and  the  movement 
leaf  at  inches  from  the  vertical       of  one  of  the  leaflets  (the  petiole 

PTem  *  2310      **      havinS  been  secured)  was  traced 


CHAP.  VII. 


SLEEP  OF   LEAVES. 


393 


during  24  h.  (Fig.  167).     The  leaf  fell  from  the  early  morning 
till  1.50  P.M.,  and  then  rose  till  6  P.M.,  when  it  was  asleep.    A 


IJ. 


C. 


Marsilea  quadrifoliata :  A,  leaf  during  the  day,  seen  from  vertically  above  ; 
B,  leaf  beginning  to  go  to  sleep,  seen  laterally ;  C,  the  same  asleep. 
Figures  reduced  to  one-half  of  natural  scale. 

vertically  dependent  glass  filament  was  now  fixed  to  one  of  the 
terminal  and  inner  leaflets ;  and  part  of  the  tracing  in  Fig.  167, 
after  6  P.M.,  shows  that  it  continued  to  sink,  making  one  zigzag, 
until  10.40  P.M.  At  6.45  A.M.  on  the  following  morning,  the  leaf 
was  awaking,  and  the  filament  pointed  above  the  vertical  glass, 

Fig.  167. 


JO'A'om. 


Marsilea  quadrifoliata :  circumnutation  and  nyctitropic  movement  of  leaflet 
traced  on  vertical  glass,  during  nearly  24  h.  Figure  reduced  to  two- 
thirds  of  original  scale.  Plant  kept  at  rather  too  low  a  temperature. 

but  by  8.25  A  M.  it  occupied  the  position  shown  in  the  figure. 
The  diagram  differs  greatly  in  appearance  from  most  of  those 
previously  given;  and  this  is  due  to  the  leaflet  twisting  and 
moving  laterally  as  it  approaches  and  comes  into  contact  with 


394  MODIFIED   CIRCUMNUTATION.  CHAP.  VIL 

its  fellow.  The  movement  of  another  leaflet,  when  asleep, 
was  traced  between  6  P.M.  and  10.35  P.M.,  and  it  clearly  cir- 
cumnutated,  for  it  continued  for  two  hours  to  sink,  then  rose, 
and  then  sank  still  lower  than  it  was  at  6  P.M.  It  in  ay  be 
seen  in  the  preceding  figure  (167)  that  the  leaflet,  when  the 
plant  was  subjected  to  a  rather  low  temperature  in  the  house, 
descended  and  ascended  during  the  middle  of  the  day  in  a 
somewhat  zigzag  line;  but  when  kept  in  the  hot-house  from 
9  A.M.  to  3  P.M.  at  a  Jiigh  but  varying  temperature  (viz.,  between 
72°  and  83°  F.)  a  leaflet  (with  the  petiole  secured)  circumnutated 
rapidly,  for  it  made  three  large  vertical  ellipses  in  the  course  of 
the  six  hours.  According  to  Brongniart,  Marsika  pubescens  sleeps 
like  the  present  species.  These  plants  are  the  sole  cryptogamic 
ones  known  to  sleep. 


Summary  and  Concluding  Remarks  on  the  Nyctitropio 
or  Sleep-movements  of  Leaves. — That  these  movements 
are  in  some  manner  of  high  importance  to  the  plants 
which  exhibit  them,  few  will  dispute  who  have  ob- 
served how  complex  they  sometimes  are.  Thus  with 
Cassia,  the  leaflets  which  are  horizontal  during  the 
day  not  only  bend  at  night  vertically  downwards  with 
the  terminal  pair  directed  considerably  backwards,  but 
they  also  rotate  on  their  own  axes,  so  that  their  lower 
surfaces  are  turned  outwards.  The  terminal  leaflet 
of  Melilotus  likewise  rotates,  by  which  movement  one 
of  its  lateral  edges  is  directed  upwards,  and  at  the 
same  time  it  moves  either  to  the  left  or  to  the  right, 
until  its  upper  surface  comes  into  contact  with  that  of 
the  lateral  leaflet  on  the  same  side,  which  has  like- 
wise rotated  on  its  own  axis.  With  Arachis,  all  four 
leaflets  form  together  during  the  night  a  single 
vertical  packet;  and  to  effect  this  the  two  anterior 
leaflets  have  to  move  upwards  and  the  two  posterior 
ones  forwards,  besides  all  twisting  on  their  own  axes. 
In  the  genus  Sida  the  leaves  of  some  species  move  at 
night  through  an  angle  of  90°  upwards,  and  of  others 


CHAP.  VII.        SUMMAKY   ON   SLEEP  OF   LEAVES.  395 

through  the  same  angle  downwards.  We  have  seen  a 
similar  difference  in  the  nyctitropic  movements  of  the 
cotyledons  in  the  genus  Oxalis.  In  Lupinus,  again, 
the  leaflets  move  either  upwards  or  downwards ;  and 
in  some  species,  for  instance  L.  luteus,  those  on  one 
side  of  the  star-shaped  leaf  move  up,  and  those  on  the 
opposite  side  move  down  ;  the  intermediate  ones  rota- 
ting on  their  axes  ;  and  by  these  varied  movements,  the 
whole  leaf  forms  at  night  a  vertical  star  instead  of  a 
horizontal  one,  as  during  the  day.  Some  leaves  and 
leaflets,  besides  moving  either  upwards  or  downwards, 
become  more  or  less  folded  at  night,  as  in  Bauhinia 
and  in  some  species  of  Oxalis.  The  positions,  indeed, 
which  leaves  occupy  when  asleep  are  almost  infinitely 
diversified ;  they  may  point  either  vertically  upwards 
or  downwards,  or,  in  the  case  of  leaflets,  towards  the 
apex  or  towards  the  base  of  the  leaf,  or  in  any  inter- 
mediate position.  They  often  rotate  at  least  as  much 
as  90°  on  their  own  axes.  The  leaves  which  arise 
from  upright  and  from  horizontal  or  much  inclined 
branches  on  the  same  plant,  move  in  some  few  cases 
in  a  different  manner,  as  with  Porlieria  and  Strephium. 
The  whole  appearance  of  many  plants  is  wonderfully 
changed  at  night,  as  may  be  seen  with  Oxalis,  and 
still  more  plainly  with  Mimosa.  A  bush  of  Acacia 
Farnesiana  appears  at  night  as  if  covered  with  little 
dangling  bits  of  string  instead  of  leaves.  Excluding 
a  few  genera  not  seen  by  ourselves,  about  which  we 
are  in  doubt,  and  excluding  a  few  others  the  leaflets  of 
which  rotate  at  night,  and  do  not  rise  or  sink  much, 
there  are  37  genera  in  which  the  leaves  or  leaflets  rise, 
often  moving  at  the  same  time  towards  the  apex  or 
towards  the  base  of  the  leaf,  and  32  genera  in  which 
they  sink  at  night. 

The  nyctitropic  movements  of  leaves,  leaflets,  and 


396  MODIFIED   CIRCUMNUTATION.  CHAP.  VII 

petioles  are  effected  in  two  different  ways ;  firstly,  by 
alternately  increased  growth  on  their  opposite '  sides, 
preceded  by  increased  turgescence  of  the  cells ;  and 
secondly  by  means  of  a  pulvinus  or  aggregate  of  small 
cells,  generally  destitute  of  chlorophyll,  which  become 
alternately  more  turgescent  on  nearly  opposite  sides ; 
and  this  turgescence  is  not  followed  by  growth  except 
during  the  early  age  of  the  plant.  A  pulvinus  seems 
to  be  formed  (as  formerly  shown)  by  a  group  of  cells 
ceasing  to  grow  at  a  very  early  age,  and  therefore  does 
not  differ  essentially  from  the  surrounding  tissues. 
The  cotyledons  of  some  species  of  Trifolium  are  pro- 
vided with  a  pulvinus,  and  others  are  destitute  of  one, 
and  so  it  is  with  the  leaves  in  the  genus  Sida.  We 
see  also  in  this  same  genus  gradations  in  the  state  of 
the  development  of  the  pulvinus ;  and  in  Nicotiana 
we  have  what  may  probably  be  considered  as  the 
commencing  development  of  one.  The  nature  of  the 
movement  is  closely  similar,  whether  a  pulvinus  is 
absent  or  present,  as  is  evident  from  many  of  the 
diagrams  given  in  this  chapter.  It  deserves  notice 
that  when  a  pulvinus  is  present,  the  ascending  and 
descending  lines  hardly  ever  coincide,  so  that  ellipses 
are  habitually  described  by  the  leaves  thus  provided, 
whether  they  are  young  or  so  old  as  to  have  quite 
ceased  growing.  This  fact  of  ellipses  being  described, 
shows  that  the  alternately  increased  turgescence  of 
the  cells  does  not  occur  on  exactly  opposite  sides  of  the 
pulvinus,  any  more  than  the  increased  growth  which 
causes  the  movements  of  leaves  not  furnished  with 
pulvini.  When  a  pulvinus  is  present,  the  nyctitropic 
movements  are  continued  for  a  very  much  longer 
period  than  when  such  do  not  exist.  This  has  been 
am.ply  proved  in  the  case  of  cotyledons,  and  Pfeffer 
has  given  observations  to  the  same  effect  with  respect 


CHAP.  VII.        SUMMARY   ON    SLEEP   OF   LEAVES.  397 

to  leaves.  We  have  seen  that  a  leaf  of  Mimosa 
pudica  continued  to  move  in  the  ordinary  manner, 
though  somewhat  more  simply,  until  it  withered  and 
died.  It  may  be  added  that  some  leaflets  of  Trifolium 
pratense  were  pinned  open  during  10  days,  and  on  the 
first  evening  after  being  released  they  rose  up  and 
slept  in  the  usual  manner.  Besides  the  long  con- 
tinuance of  the  movements  when  effected  by  the  aid 
of  a  pulvinus  (and  this  appears  to  be  the  final  cause 
of  its  development),  a  twisting  movement  at  night,  as 
Pfeffer  has  remarked,  is  almost  confined  to  leaves  thus 
provided. 

It  is  a  very  general  rule  that  the  first  true  leaf, 
though  it  may  differ  somewhat  in  shape  from  the 
leaves  on  the  mature  plant,  yet  sleeps  like  them  ;  and 
this  occurs  quite  independently  of  the  fact  whether  or 
not  the  cotyledons  themselves  sleep,  or  whether  they 
sleep  in  the  same  manner.  But  with  Phaseolus  Eox- 
lurghii  the  first  unifoliate  leaves  rise  at  night  almost 
sufficiently  to  be  said  to  sleep,  whilst  the  leaflets  of 
the  secondary  trifoliate  leaves  sink  vertically  at  night. 
On  young  plants  of  Sida  rhomb&folia,  only  a  few 
inches  in  height,  the  leaves  did  not  sleep,  though  on 
rather  older  plants  they  rose  up  vertically  at  night. 
On  the  other  hand,  the  leaves  on  very  young  plants  of 
Cytisus  fragrans  slept  in  a  conspicuous  manner,  whilst 
on  old  and  vigorous  bushes  kept  in  the  greenhouse, 
the  leaves  did  not  exhibit  any  plain  nyctitropic  move- 
ment. In  the  genus  Lotus  the  basal  stipule-like 
leaflets  rise  up  vertically  at  night,  and  are  provided 
with  pulvini. 

As  already  remarked,  when  leaves  or  leaflets  change 
their  position  greatly  at  night  and  by  complicated 
movements,  it  can  hardly  be  doubted  that  these  must 
be  in  some  manner  beneficial  to  the  plant.  If  so,  we 


398  MODIFIED  CIECUMNUTATIOX.  CHAP.  VII. 

must  extend  the  same  conclusion  to  a  large  number  of 
sleeping  plants;  for  the  most  complicated  and  the 
simplest  nyctitropic  movements  are  connected  together 
by  the  finest  gradations.  But  owing  to  the  causes  spe- 
cified in  the  beginning  of  this  chapter,  it  is  impossible 
in  some  few^ases  to  determine  whether  or  not  certain 
movements  should  be  called  nyctitropic.  Generally, 
the  position  which  the  leaves  occupy  at  night  indi- 
cates with  sufficient  clearness,  that  the  benefit  thus 
derived,  is  the  protection  of  their  upper  surfaces  from 
radiation  into  the  open  sky,  and  in  many  cases  the 
mutual  protection  of  all  the  parts  from  cold  by  their 
being  brought  into  close  approximation.  It  should  be 
remembered  that  it  was  proved  in  the  last  chapter,  that 
leaves  compelled  to  remain  extended  horizontally  at 
night,  suffered  much  more  from  radiation  than  those 
which  were  allowed  to  assume  their  normal  vertical 
position. 

The  fact  of  the  leaves  of  several  plants  not  sleeping 
unless  they  have  been  well  illuminated  during  the 
day,  made  us  for  a  time  doubt  whether  the  pro- 
tection of  their  upper  surfaces  from  radiation  was  in 
all  cases  the  final  cause  of  their  well-pronounced 
nyctitropic  movements.  But  we  have  no  reason  to 
suppose  that  the  illumination  from  the  open  sky, 
during  even  the  most  clouded  day,  is  insufficient  for 
this  purpose ;  and  we  should  bear  in  mind  that  leaves 
which  are  shaded  from  being  seated  low  down  on  the 
plant,  and  which  sometimes  do  not  sleep,  are  likewise 
protected  at  night  from  full  radiation.  Nevertheless, 
we  do  not  wish  to  deny  that  there  may  exist  cases  in 
which  leaves  change  their  position  considerably  at 
night,  without  their  deriving  any  benefit  from  such 
movements. 

Although  with  sleeping   plants  the  blades  almost 


CHAP.  VII.       SUMMAEY   ON   SLEEP   OF  LEAVES.  399 

always  as«ume  at  night  a  vertical,  or  nearly  vertical 
position,  it  is  a  point  of  complete  indifference  whether 
the  apex,  or  the  base,  or  one  of  the  lateral  edges,  is 
directed  to  the  zenith.  It  is  a  rule  of  wide  generality, 
than  whenever  there  is  any  difference  in  the  degree  of 
exposure  to  radiation  between  the  upper  and  the  lower 
surfaces  of  leaves  and  leaflets,  it  is  the  upper  which  is 
the  least  exposed,  as  may  be  seen  in  Lotus,  Cytisus, 
Trifolium,  and  other  genera.  In  several  species  of 
Lupinus  the  leaflets  do  not,  and  apparently  from 
their  structure  cannot,  place  themselves  vertically  at 
night,  and  consequently  their  upper  surfaces,  though 
highly  inclined,  are  more  exposed  than  the  lower ;  and 
here  we  have  an  exception  to  our  rule.  But  in  other 
species  of  this  genus  the  leaflets  succeed  in  placing 
themselves  vertically ;  this,  however,  is  effected  by  a 
very  unusual  movement,  namely,  by  the  leaflets  on 
the  opposite  sides  of  the  same  leaf  moving  in  opposite 
directions. 

It  is  again  a  very  common  rule  that  when  leaflets 
come  into  close  contact  with  one  another,  they  do  so 
by  their  upper  surfaces,  which  are  thus  best  protected. 
In  some  cases  this  may  be  the  direct  result  of  their 
rising  vertically ;  but  it  is  obviously  for  the  pro- 
tection of  the  upper  surfaces  that  the  leaflets  of 
Cassia  rotate  in  so  wonderful  a  manner  whilst  sinking 
downwards  ;  and  that  the  terminal  leaflet  of  Melilotus 
rotates  and  moves  to  one  side  until  it  meets  the  lateral 
leaflet  on  the  same  side.  When  opposite  leaves  or 
leaflets  sink  vertically  down  without  any  twisting, 
their  lower  surfaces  approach  each  other  and  some- 
times come  into  contact;  but  this  is  the  direct  and 
inevitable  result  of  their  position.  With  many  species 
of  Oxalis  the  lower  surfaces  of  the  adjoining  leaflets 
are  pressed  together,  and  are  thus  better  protected 
18 


400  MODIFIED  CIRCUMNUTATION.  CHAP.  VII. 

than  the  upper  surfaces ;  but  this  depends  merely  on 
each  leaflet  becoming  folded  at  night  so  as  to  be  able 
to  sink  vertically  downwards.  The  torsion  or  rotation 
of  leaves  and  leaflets,  which  occurs  in  so  many  cases, 
apparently  always  serves  to  bring  their  upper  surfaces 
into  close  approximation  with  one  another,  or  with 
other  parts  of  the  plant,  for  their  mutual  protection. 
We  see  this  best  in  such  cases  as  those  of  Arachis, 
Mimosa  albida,  and  Marsilea,  in  which  all  the  leaflets 
form  together  at  night  a  single  vertical  packet.  If 
with  Mimosa  pudica  the  opposite  leaflets  had  merely 
moved  upwards,  their  upper  surfaces  would  have  come 
into  contact  and  been  well  protected ;  but  as  it  is, 
they  all  successively  move  towards  the  apex  of  the 
leaf ;  and  thus  not  only  their  upper  surfaces  are  pro- 
tected, but  the  successive  pairs  become  imbricated  and 
mutually  protect  one  another  as  well  as  the  petioles. 
This  imbrication  of  the  leaflets  of  sleeping  plants  is  a 
common  phenomenon. 

The  nyctitropic  movement  of  the  blade  is  gene- 
rally effected  by  the  curvature  of  the  uppermost  part 
of  the  petiole,  which  has  often  been  modified  into  a 
pulvinus ;  or  the  whole  petiole,  when  short,  may  be 
thus  modified.  But  the  blade  itself  sometimes  curves 
or  moves,  of  which  fact  Bauhinia  offers  a  striking 
instance,  as  the  two  halves  rise  up  and  come  into 
close  contact  at  night.  Or  the  blade  and  the  upper 
part  of  the  petiole  may  both  move.  Moreover,  the 
petiole  as  a  whole  commonly  either  rises  or  sinks  at 
night.  This  movement  is  sometimes  large :  thus  the 
petioles  of  Cassia  pubescens  stand  only  a  little  above 
the  horizon  during  the  day,  and  at  night  rise  up 
almost,  or  quite,  perpendicularly.  The  petioles  of  the 
younger  leaves  of  Desmodium  gyrans  also  rise  up  ver- 
tically at  night.  On  the  other  hand,  with  Amphi- 


CHAP.  VII.       SUMMARY  ON  SLEEP   OF   LEAVES.  401 

carpaea,  the  petioles  of  some  leaves  sank  down  as 
much  as  57°  at  night;  with  Arachis  they  sank  39°, 
and  then  stood  at  right  angles  to  the  stem.  Gene- 
rally, when  the  rising  or  sinking  of  several  petioles  on 
the  same  plant  was  measured,  the  amount  differed 
greatly.  This  is  largely  determined  by  the  age  of  the 
leaf :  for  instance,  the  petiole  of  a  moderately  old  leaf 
of  Desmodium  gyrans  rose  only  46°,  whilst  the  young 
ones  rose  up  vertically ;  that  of  a  young  leaf  of  Cassia 
floribunda  rose  41°,  whilst  that  of  an  older  leaf  rose 
only  12°.  It  is  a  more  singular  fact  that  the  age  of 
the  plant  sometimes  influences  greatly  the  amount  of 
movement ;  thus  with  some  young  seedlings  of  a  Bau- 
hinia  the  petioles  rose  at  night  30°  and  34°,  whereas 
those  on  these  same  plants,  when  grown  to  a  height 
of  2  or  3  feet,  hardly  moved  at  all.  The  position  of 
the  leaves  on  the  plant  as  determined  by  the  light, 
seems  also  to  influence  the  amount  of  movement 
of  the  petiole;  for  no  other  cause  was  apparent 
why  the  petioles  of  some  leaves  of  Melilotus  officinalis 
rose  as  much  as  59°,  and  others  only  7°  and  9°  at 
night. 

In  the  case  of  many  plants,  the  petioles  move  at 
night  in  one  direction  and  the  leaflets  in  a  directly 
opposite  one.  Thus,  in  three  genera  of  Phaseolese  the 
leaflets  moved  vertically  downwards  at  night,  and  the 
petioles  rose  in  two  of  them,  whilst  in  the  third  they 
sank.  Species  in  the  same  genus  often  differ  widely 
in  the  movements  of  their  petioles.  Even  on  the  same 
plant  of  Lupinus pubescens  some  of  the  petioles  rose  30°, 
others  only  6°,  and  others  sank  4°  at  night.  The 
leaflets  of  Cassia  Barclayana  moved  so  little  at  night 
that  they  could  not  be  said  to  sleep,  yet  the  petioles 
of  some  young  leaves  rose  as  much  as  34°.  These 
several  facts  apparently  indicate  that  the  movements 


402  MODIFIED  CI11CUMNUTATION.          CHAP.  VII. 

of  the  petioles  are  not  performed  for  any  special  pur- 
pose; though  a  conclusion  of  this  kind  is  generally 
rash.  When  the  leaflets  sink  vertically  down  at  night 
and  the  petioles  rise,  as  often  occurs,  it  is  certain  that 
the  upward  movement  of  the  latter  does  not  aid  the 
leaflets  in  placing  themselves  in  their  proper  posi- 
tion at  night,  for  they  have  to  move  through  a 
greater  angular  space  than  would  otherwise  have  been 
necessary. 

Notwithstanding  what  has  just  been  said,  it  may  be 
strongly  suspected  that  in  some  cases  the  rising  of 
the  petioles,  wrhen  considerable,  does  beneficially  serve 
the  plant  by  greatly  reducing  the  surface  exposed  to 
radiation  at  night.  If  the  reader  will  compare  the 
two  drawings  (Fig.  155,  p.  371)  of  Cassia  pubescens, 
copied  from  photographs,  he  will  see  that  the  dia- 
meter of  the  plant  at  night  is  about  one-third  of 
what  it  is  by  day,  and  therefore  the  surface  exposed 
to  radiation  is  nearly  nine  times  less.  A  similar 
conclusion  may  be  deduced  from  the  drawings  (Fig. 
149,  p.  358)  of  a  branch  awake  and  asleep  of  Des- 
modium  gyrans.  So  it  was  in  a  very  striking  manner 
with  young  plants  of  Bauhinia,  and  with  Oxalis 
Ortegesii. 

We  are  led  to  an  analogous  conclusion  with  respect 
to  the  movements  of  the  secondary  petioles  of  certain 
pinnate  leaves.  The  pinnae  of  Mimosa  pudica  con- 
verge at  night;  and  thus  the  imbricated  and  closed 
leaflets  on  each  separate  pinna  are  all  brought  close 
together  into  a  single  bundle,  and  mutually  protect 
one  another,  with  a  somewhat  smaller  surface  exposed 
to  radiation.  With  Albizzia  lopliantha  the  pinnae  close 
together  in  the  same  manner.  Although  the  pinnae 
of  Acacia  Farnesiana  do  not  converge  much,  they 
sink  downwards.  Those  of  Neptunia  oleracea  likewise 


CHAP.  VII.       SUMMARY  ON   SLEEP  OF  LEAVES.          403 

move  downwards,  as  well  as  backwards,  towards  the 
base  of  the  leaf,  whilst  the  main  petiole  rises.  With 
Schrankia,  again,  the  pinnae  are  depressed  at  night. 
Now  in  these  three  latter  cases,  though  the  pinnae 
do  not  mutually  protect  one  another  at  night,  yet 
after  having  sunk  down  they  expose,  as  does  a 
dependent  sleeping  leaf,  much  less  surface  to  the 
zenith  and  to  radiation  than  if  they  had  remained 
horizontal. 

Any  one  who  had  never  observed  continuously  a 
sleeping  plant,  would  naturally  suppose  that  the  leaves 
moved  only  in  the  evening  when  going  to  sleep,  and 
in  the  morning  when  awaking ;  but  he  would  be  quite 
mistaken,  for  we  have  found  no  exception  to  the  rule 
that  leaves  which  sleep  continue  to  move  during  the 
whole  twenty-four  hours;  they  move,  however,  more 
quickly  when  going  to  sleep  and  when  awaking  than 
at  other  times.  That  they  are  not  stationary  during 
the  day  is  shown  by  all  the  diagrams  given,  and  by 
the  many  more  which  were  traced.  It  is  troublesome 
to  observe  the  movements  of  leaves  in  the  middle  of 
the  night,  but  this  was  done  in  a  few  cases;  and 
tracings  were  made  during  the  early  part  of  the  night 
of  the  movements,  in  the  case  of  Oxalis,  Amphicarpaea, 
two  species  of  Erythrina,  a  Cassia,  Passiflora,  Euphorbia 
and  Marsilea ;  and  the  leaves  after  they  had  gone  to 
sleep,  were  found  to  be  in  constant  movement.  When, 
however,  opposite  leaflets  come  into  close  contact  with 
one  another  or  with  the  stem  at  night,  they  are,  as  we 
believe,  mechanically  prevented  from  moving,  but  this 
point  was  not  sufficiently  investigated. 

When  the  movements  of  sleeping  leaves  are  traced 
during  twenty-four  hours,  the  ascending  and  descend- 
ing lines  do  not  coincide,  except  occasionally  and  by 
accident  for  a  short  space  ;  so  that  with  many  plants  a 


404  MODIFIED   CIRCUMNUTATION.  CHAP.  VII. 

single  large  ellipse  is  described  during  each  twenty-four 
hours.  Such  ellipses  are  generally  narrow  and  ver- 
tically directed,  for  the  amount  of  lateral  movement  is 
small.  That  there  is  some  lateral  movement  is  shown 
by  the  ascending  and  descending  lines  not  coinciding, 
and  occasionally,  as  with  Desmodium  gyrans  and  Thalia 
dealbata,  it  was  strongly  marked.  In  the  case  of  Meli- 
lotus  the  ellipses  described  by  the  terminal  leaflet 
during  the  day  are  laterally  extended,  instead  of  ver- 
tically, as  is  usual ;  and  this  fact  evidently  stands  in 
relation  with  the  terminal  leaflet  moving  laterally 
when  it  goes  to  sleep.  With  the  majority  of  sleeping 
plants  the  leaves  oscillate  more  than  once  up  and 
down  in  the  twenty-four  hours ;  so  that  frequently  two 
ellipses,  one  of  moderate  size,  and  one  of  very  large  size 
which  includes  the  nocturnal  movement,  are  described 
within  the  twenty-four  hours.  For  instance,  a  leaf 
which  stands  vertically  up  during  the  night  will  sink 
in  the  morning,  then  rise  considerably,  again  sink  in 
the  afternoon,  and  in  the  evening  reascend  and  assume 
its  vertical  nocturnal  position.  It  will  thus  describe, 
in  the  course  of  the  twenty-four  hours,  two  ellipses  of 
unequal  sizes.  Other  plants  describe  within  the  same 
time,  three,  four,  or  five  ellipses.  Occasionally  the 
longer  axes  of  the  several  ellipses  extend  in  different 
directions,  of  which  Acacia  Farnesiana  offered  a  good 
instance.  The  following  cases  will  give  an  idea  of  the 
rate  of  movement :  Oxalis  acetosella  completed  two 
ellipses  at  the  rate  of  1  h.  25  m.  for  each ;  Marsilea 
quadrifoliata,  at  the  rate  of  2  h.;  Trifolium  subterraneum, 
one  in  3  h.  30  rn. ;  and  Aracliis  liypogsea,  in  4  h.  50  m. 
But  the  number  of  ellipses  described  within  a  given 
time  depends  largely  on  the  state  of  the  plant  and 
on  the  conditions  to  which  it  is  exposed.  It  often  hap- 
pens that  a  single  ellipse  may  be  described  during  one 


CHAP.  VII.       SUMMARY   ON   SLEEP  OF  LEAVES.          405 

day,  and  two  on  the  next.  Erythrina  corallodendron 
made  four  ellipses  on  the  first  day  of  observation 
and  only  a  single  one  on  the  third,  apparently  owing 
to  having  been  kept  not  sufficiently  illuminated  and 
perhaps  not  warm  enough.  But  there  seems  likewise 
to  be  an  innate  tendency  in  different  species  of  the 
same  genus  to  make  a  different  number  of  ellipses  in 
the  twenty-four  hours :  the  leaflets  of  Trifolium  repens 
made  only  one ;  those  of  T.  resupinatum  two,  and  those 
of  T.  subterraneum  three  in  this  time.  Again,  the 
leaflets  of  Oxalis  Plumierii  made  a  single  ellipse ;  those 
of  0.  lupleurifolia,  two ;  those  of  0.  Valdiviana,  two  or 
three ;  and  those  of  0.  acetosella,  at  least  five  in  the 
twenty-four  hours. 

The  line  followed  by  the  apex  of  a  leaf  or  leaflet, 
whilst  describing  one  or  more  ellipses  during  the  day, 
is  often  zigzag,  either  throughout  its  whole  course  or 
only  during  the  morning  or  evening :  Robinia  offered 
an  instance  of  zigzagging  confined  to  the  morning, 
and  a  similar  movement  in  the  evening  is  shown  in 
the  diagram  (Fig.  126)  given  under  Sida.  The  amount 
of  the  zigzag  movement  depends  largely  on  the  plant 
being  placed  under  highly  favourable  conditions.  But 
even  under  such  favourable  conditions,  if  the  dots  which 
mark  the  position  of  the  apex  are  made  at  consider- 
able intervals  of  time,  and  the  dots  are  then  joined, 
the  course  pursued  will  still  appear  comparatively 
simple,  although  the  number  of  the  ellipses  will  be 
increased;  but  if  dots  are  made  every  two  or  three 
minutes  and  these  are  joined,  the  result  often  is  that 
all  the  lines  are  strongly  zigzag,  many  small  loops, 
triangles,  and  other  figures  being  also  formed.  This 
fact  is  shown  in  two  parts  of  the  diagram  (Fig.  150) 
of  the  movements  of  Desmodium  gyrans.  Strephium 
Jloribundum,  observed  under  a  high  temperature, 


406  MODIFIED  CIRCUMNUTATIOX.          CHAP.  VII. 

made  several  little  triangles  at  the  rate  of  43  m. 
for  each.  Mimosa  pudica,  similarly  observed,  de- 
scribed three  little  ellipses  in  67  m. ;  and  the  apex 
of  a  leaflet  crossed  ^^  of  an  inch  in  a  second,  or 
O12  inch  in  a  minute.  The  leaflets  of  Averrhoa 
made  a  countless  number  of  little  oscillations  when 
the  temperature  was  high  and  the  sun  shining.  The 
zigzag  movement  may  in  all  cases  be  considered  as 
an  attempt  to  form  small  loops,  which  are  drawn  out 
by  a  prevailing  movement  in  some  one  direction.  The 
rapid  gyrations  of  the  little  lateral  leaflets  of  Des- 
modium  belong  to  the  same  class  of  movements, 
somewhat  exaggerated  in  rapidity  and  amplitude. 
The  jerking  movements,  with  a  small  advance  and 
still  smaller  retreat,  apparently  not  exactly  in  the 
same  line,  of  the  hypocotyl  of  the  cabbage  and  of 
the  leaves  of  Dionaea,  as  seen  under  the  microscope, 
all  probably  come  under  this  same  head.  We  may 
suspect  that  we  here  see  the  energy  which  is  freed 
during  the  incessant  chemical  changes  in  progress  in 
the  tissues,  converted  into  motion.  Finally,  it  should 
be  noted  that  leaflets  and  probably  some  leaves,  whilst 
describing  their  ellipses,  often  rotate  slightly  on  their 
axes ;  so  that  the  plane  of  the  leaf  is  directed  first  to 
one  and  then  to  another  side.  This  was  plainly  seen 
to  be  the  case  with  the  large  terminal  leaflets  of  Des- 
modium,  Erythrina  and  Amphicarpaea,  and  is  probably 
common  to  all  leaflets  provided  with  a  pulvinus. 

With  respect  to  the  periodicity  of  the  movements  of 
sleeping  leaves,  Pfeffer*  has  so  clearly  shown  that 
this  depends  on  the  daily  alternations  of  light  and 
darkness,  that  nothing  farther  need  be  said  on  this 


*  'Die  Periodischen  Bewcgungen  der  Blattorgane,'  1875,  p.  30,  et 
passim. 


CHAP.  VII.        SUMMARY   ON   SLEEP   OF   LEAVES.  407 

head.  But  we  may  recall  the  behaviour  of  Mimosa 
in  the  North,  where  the  sun  does  not  set,  and  the 
complete  inversion  of  the  daily  movements  by  artificial 
light  and  darkness.  It  has  also  been  shown  by  us, 
that  although  leaves  subjected  to  darkness  for  a  mode- 
rately long  time  continue  to  circumnutate,  yet  the 
periodicity  of  their  movements  is  soon  greatly  dis- 
turbed, or  quite  annulled.  The  presence  of  light  or 
its  absence  cannot  be  supposed  to  be  the  direct  cause 
of  the  movements,  for  these  are  wonderfully  diversified 
even  with  the  leaflets  of  the  same  leaf,  although  all 
have  of  course  been  similarly  exposed.  The  move- 
ments depend  on  innate  causes,  and  are  of  an  adaptive 
nature.  The  alternations  of  light  and  darkness 
merely  give  notice  to  the  leaves  that  the  period  has 
arrived  for  them  to  move  in  a  certain  manner.  We 
may  infer  from  the  fact  of  several  plants  (Tropoeolum, 
Lupinus,  &c.)  not  sleeping  unless  they  have  been  well 
illuminated  during  the  day,  that  it  is  not  the  actual 
decrease  of  light  in  the  evening,  but  the  contrast 
between  the  amount  at  this  hour  and  during  the  early 
part  of  the  day,  which  excites  the  leaves  to  modify 
their  ordinary  mode  of  circumnutation. 

As  the  leaves  of  most  plants  assume  their  proper 
diurnal  position  in  the  morning,  although  ligh{  be 
excluded,  and  as  the  leaves  of  some  plants  continue  to 
move  in  the  normal  manner  in  darkness  during  at 
least  a  whole  day,  we  may  conclude  that  the  periodi- 
city of  their  movements  is  to  a  certain  extent  in- 
herited.* The  strength  of  such  inheritance  differs 


*  Pfeffer  denies   such  inherit-  "  Nachwirkung,"    or    <he    after- 

ance ;  he  attributes  ('  Die  Period.  effects    of    light    and    darkness. 

Bewegungen,'    pp.     30-56)     the  But  we  are  unable  to  follow  his 

periodicity    when    prolonged  for  train  of  reasoning.     There  does 

a  day  or    two   in    darkness,  to  not  seern  to  be  any  more  reason  for 


408  MODIFIED   CIKCUMNUTATION.  CHAP.  VII. 

much  in  different  species,  and  seems  never  to  be  rigid ; 
for  plants  have  been  introduced  from  all  parts  of  the 
world  into  our  gardens  and  greenhouses ;  and  if  their 
movements  had  been  at  all  strictly  fixed  in  relation  to 
the  alternations  of  day  and  night,  they  would  have 
slept  in  this  country  at  very  different  hours,  which 
is  not  the  case.  Moreover,  it  has  been  observed  that 
sleeping  plants  in  their  native  homes  change  their 
times  of  sleep  Avith  the  changing  seasons.  * 

We  may  now  turn  to  the  systematic  list  (p.  320). 
This  contains  the  names  of  all  the  sleeping  plants 
known  to  us,  though  the  list  undoubtedly  is  very 
imperfect.  It  may  be  premised  that,  as  a  general 
rule,  all  the  species  in  the  same  genus  sleep  in 
nearly  the  same  manner.  But  there  are  some  ex- 
ceptions; in  several  large  genera  including  many 
sleeping  species  (for  instance,  Oxalis),  some  do  not 
sleep.  One  species  of  Melilotus  sleeps  like  a  Tri- 
folium,  and  therefore  very  differently  from  its  con- 
geners ;  so  does  one  species  of  Cassia.  In  the  genus 
Sida,  the  leaves  either  rise  or  fall  at  night ;  and  with 
Lupinus  they  sleep  in  three  different  methods.  Re- 
turning to  the  list,  the  first  point  which  strikes  us,  is 
that  there  are  many  more  genera  amongst  the  Legu- 
rninosae  (and  in  almost  every  one  of  the  Leguminous 
tribes)  than  in  all  the  other  families  put  together; 
and  we  are  tempted  to  connect  this  fact  with  the  great 


attributing  such  movements  to  this  effect  must  be  produced   on  the 
cause  than,  for  instance,  the  in-  seeds  by  the  long-continued  culti- 
herited    habit     of     winter     and  vation  of  the  parent-plants  under 
summer  wheat   to  grew   best  at  different  climates,  but  no  one  pro- 
different   seasons  ;  tor  this   habit  bably  would  call  this  the  "  Nach- 
is  lost  after  a  few  years,  like  the  wirkung  "  of  the  climates, 
movements  of  leaves  in  darkness  *  Pfeffer,  ibid.,  p.  46. 
after  a  few  days.    No  doubt  some 


CHAP.  VII.       SUMMARY   ON   SLEEP  OF   LEAVES.  409 

mobility  of  the  stems  and  leaves  in  this  family,  as 
shown  by  the  large  number  of  climbing  species  which 
it  contains.  Next  to  the  Leguminosae  come  the  Mal- 
vaceae, together  with  some  closely  allied  families.  But 
by  far  the  most  important  point  in  the  list,  is  that  we 
meet  with  sleeping  plants  in  28  families,  in  all  the 
great  divisions  of  the  Phanerogamic  series,  and  in  one 
Cryptogam.  Now,  although  it  is  probable  that  with 
the  Leguminosoe  the  tendency  to  sleep  may  have  been 
inherited  from  one  or  a  few  progenitors,  and  possibly 
so  in  the  cohorts  of  the  Malvales  and  Chenopodiales, 
yet  it  is  manifest  that  the  tendency  must  have  been 
acquired  by  the  several  genera  in  the  other  families, 
quite  independently  of  one  another.  Hence  the  ques- 
tion naturally  arises,  how  has  this  been  possible  ? 
and  the  answer,  we  cannot  doubt,  is  that  leaves  owe 
their  nyctitropic  movements  to  their  habit  of  cir- 
cumnutating,  —  a  habit  common  to  all  plants,  and 
everywhere  ready  for  any  beneficial  development  or 
modification. 

It  has  been  shown  in  the  previous  chapters  that  the 
leaves  and  cotyledons  of  all  plants  are  continually 
moving  up  and  down,  generally  to  a  slight  but  some- 
times to  a  considerable  extent,  and  that  they  describe 
either  one  or  several  ellipses  in  the  course  of  twenty- 
four  hours ;  they  are  also  so  far  affected  by  the  alter- 
nations of  day  and  night  that  they  generally,  or 
at  least  often,  move  periodically  to  a  small  extent; 
and  here  we  have  a  basis  for  the  development  of  the 
greater  nyctitropic  movements.  That  the  movements 
of  leaves  and  cotyledons  which  do  not  sleep  come 
within  the  class  of  circumnutating  movements  cannot 
be  doubted,  for  they  are  closely  similar  to  those  of 
hypocotyls,  epicotyls,  the  stems  of  mature  plants,  and 
of  various  other  organs.  Now,  if  we  take  the  simplest 


410  MODIFIED  CIRCUMNUTATION.          CHAP.  VII. 

case  of  a  sleeping  leaf,  we  see  that  it  makes  a  single 
ellipse  in  the  twenty-four  hours,  which  resembles  one 
described  by  a  non-sleeping  leaf  in  every  respect,  except 
that  it  is  much  larger.  In  both  cases  the  course  pursued 
is  often  zigzag.  As  all  non-sleeping  leaves  are  inces- 
santly circumnutating,  we  must  conclude  that  a  part 
at  least  of  the  upward  and  downward  movement  of  one 
that  sleeps,  is  due  to  ordinary  circumnutation ;  and  it 
seems  altogether  gratuitous  to  rank  the  remainder  of 
the  movement  under  a  wholly  different  head.  With 
a  multitude  of  climbing  plants  the  ellipses  which  they 
describe  have  been  greatly  increased  for  another  pur- 
pose, namely,  catching  hold  of  a  support.  With  these 
climbing  plants,  the  various  circumnutating  organs  have 
been  so  far  modified  in  relation  to  light  that,  differently 
from  all  ordinary  plants,  they  do  not  bend  towards  it. 
With  sleeping  plants  the  rate  and  amplitude  of  the 
movements  of  the  leaves  have  been  so  far  modified  in 
relation  to  light,  that  they  move  in  a  certain  direction 
with  the  waning  light  of  the  evening  and  with  the 
increasing  light  of  the  morning  more  rapidly,  and  to 
a  greater  extent,  than  at  other  hours 

But  the  leaves  and  cotyledons  of  many  non-sleeping 
plants  move  in  a  much  more  complex  manner  than  in 
the  cases  just  alluded  to,  for  they  describe  two,  three, 
or  more  ellipses  in  the  course  of  a  day.  Now,  if  a 
plant  of  this  kind  were  converted  into  one  that  slept, 
one  side  of  one  of  the  several  ellipses  which  each 
leaf  daily  describes,  would  have  to  be  greatly  increased 
in  length  in  the  evening,  until  the  leaf  stood  ver- 
tically, when  it  would  go  on  circumnutating  about  the 
same  spot.  On  the  following  morning,  the  side  of 
another  ellipse  would  have  to  be  similarly  increased 
in  length,  so  as  to  bring  the  leaf  back  again  into  its 
diurnal  position,  when  it  would  again  circumnutate 


CHAP.  VII.       SUMMARY  ON   SLEEP  OF   LEAVES.  411 

until  the  evening.  If  the  reader  will  look,  for  in- 
stance, at  the  diagram  (Fig.  142,  p.  351),  representing 
the  nyctitropic  movements  of  the  terminal  leaflet  of 
Trifolium  subterraneum,  remembering  that  the  curved 
broken  lines  at  the  top  ought  to  be  prolonged  much 
higher  up,  he  will  see  that  the  great  rise  in  the  evening 
and  the  great  fall  in  the  morning  together  form  a 
large  ellipse  like  one  of  those  described  during  the 
daytime,  differing  only  in  size.  Or,  he  may  look  at 
the  diagram  (Fig.  103,  p.  236)  of  the  3J  ellipses 
described  in  the  course  of  6  h.  35  m.  by  a  leaf  of 
Lupinus  speciosus,  which  is  one  of  the  species  in  this 
genus  that  does  not  sleep  ;  and  he  will  see  that  by 
merely  prolonging  upwards  the  line  which  was  already 
rising  late  in  the  evening,  and  bringing  it  down 
again  next  morning,  the  diagram  would  represent  the 
movements  of  a  sleeping  plant. 

With  those  sleeping  plants  which  describe  several 
ellipses  in  the  daytime,  and  which  travel  in  a  strongly 
zigzag  line,  often  making  in  their  course  minute  loops, 
triangles,  &c.,  if  as  soon  as  one  of  the  ellipses  begins 
in  the  evening  to  be  greatly  increased  in  size,  dots  are 
made  every  2  or  3  minutes  and  these  are  joined,  the 
line  then  described  is  almost  strictly  rectilinear,  in 
strong  contrast  with  the  lines  made  during  the  day- 
time. This  was  observed  with  Desmodium  gyrans  and 
Mimosa  pudica.  With  this  latter  plant,  moreover,  the 
pinnae  converge  in  the  evening  by  a  steady  move- 
ment, whereas  during  the  day  they  are  continually 
converging  and  diverging  to  a  slight  extent.  In  all 
such  cases  it  was  scarcely  possible  to  observe  the 
difference  in  the  movement  during  the  day  and  even- 
ing, without  being  convinced  that  in  the  evening  the 
plant  saves  the  expenditure  of  force  by  not  moving 
laterally,  and  that  its  whole  energy  is  now  expended 


412  MODIFIED   CIRCUMNUTATION.  CHAP.  VII 

in  gaining  quickly  its  proper  nocturnal  position  by 
a  direct  course.  In  several  other  cases,  for  instance, 
when  a  leaf  after  describing  during  the  day  one  or 
more  fairly  regular  ellipses,  zigzags  much  in  the 
evening,  it  appears  as  if  energy  was  being  expended, 
so  that  the  great  evening  rise  or  fall  might  coin- 
cide with  the  period  of  the  day  proper  for  this 
movement. 

The  most  complex  of  all  the  movements  performed 
by  sleeping  plants,  is  that  when  leaves  or  leaflets, 
after  describing  in  the  daytime  several  vertically 
directed  ellipses,  rotate  greatly  on  their  axes  in  the 
evening,  by  which  twisting  movement  they  occupy 
a  wholly  different  position  at  night  to  what  they  do 
during  the  day.  For  instance,  the  terminal  leaflets 
of  Cassia  not  only  move  vertically  downwards  in  the 
evening,  but  twist  round,  so  that  their  lower  surfaces 
face  outwards.  Such  movements  are  wholly,  or  almost 
wholly,  confined  to  leaflets  provided  with  a  pulvinus. 
But  this  torsion  is  not  a  new  kind  of  movement 
introduced  solely  for  the  purpose  of  sleep;  for  it 
has  been  shown  that  some  leaflets  whilst  describing 
their  ordinary  ellipses  during  the  daytime  rotate 
slightly,  causing  their  blades  to  face  first  to  one  side 
and  then  to  another.  Although  we  can  see  how  the 
slight  periodical  movements  of  leaves  in  a  vertical 
plane  could  be  easily  converted  into  the  greater  yet 
simple  nyctitropic  movements,  we  do  not  at  present 
know  by  what  graduated  steps  the  more  complex 
movements,  effected  by  the  torsion  of  the  pulvini, 
have  been  acquired.  A  probable  explanation  could 
be  given  in  each  case  only  after  a  close  investigation 
of  the  movements  in  all  the  allied  forms. 

From  the  facts  and  considerations  now  advanced  we 
may  conclude  that  nyctitropism,  or  the  sleep  of  leaves 


CHAF.  VII.  MODIFIED   CIRCUMNUTATION.  413 

and  cotyledons,  is  merely  a  modification  of  their  ordi- 
nary circumnutating  movement,  regulated  in  its  period 
and  amplitude  by  the  alternations  of  light  and  dark- 
ness. The  object  gained  is  the  protection  of  the  upper 
surfaces  of  the  leaves  from  radiation  at  night,  often 
combined  with  the  mutual  protection  of  the  several 
parts  by  their  close  approximation.  In  such  cases  as 
those  of  the  leaflets  of  Cassia — of  the  terminal  leaflets 
of  Melilotus — of  all  the  leaflets  of  Arachis,  Marsilea, 
&c. — we  have  ordinary  circumnutation  modified  to  the 
extreme  extent  known  to  us  in  any  of  the  several  great 
classes  of  modified  circumnutation.  On  this  view  of 
the  origin  of  nyctitropism  we  can  understand  how  it 
is  that  a  few  plants,  widely  distributed  throughout  the 
Vascular  series,  have  been  able  to  acquire  the  habit  of 
placing  the  blades  of  their  leaves  vertically  at  night, 
that  is,  of  sleeping, — a  fact  otherwise  inexplicable. 

The  leaves  of  some  plants  move  during  the  day  in 
a  manner,  which  has  improperly  been  called  diurnal 
sleep ;  for  when  the  sun  shines  brightly  on  them,  they 
direct  their  edges  towards  it.  To  such  cases  we  shall 
recur  in  the  following  chapter  on  Heliotropism.  It 
has  been  shown  that  the  leaflets  of  one  form  of 
Porlieria  liygrometrica  keep  closed  during  the  day,  as 
long  as  the  plant  is  scantily  supplied  with  water,  in 
the  same  manner  as  when  asleep ;  and  this  apparently 
serves  to  check  evaporation.  There  is  only  one  other 
analogous  case  known  to  us,  namely,  that  of  certain 
Graminese,  which  fold  inwards  the  sides  of  their  narrow 
leaves,  when  these  are  exposed  to  the  sun  and  to  a 
dry  atmosphere,  as  described  by  Duval-Jouve.*  We 
have  also  observed  the  same  phenomenon  in  Elymus 
arenareus. 


*  » Annal.  des  Sc.  Nat.  (Bot.)/  1875,  torn.  i.  pp.  326-329. 


414  STEUCTUEE   OF  CHAP.  VII. 

There  is  another  movement,  which  since  the  time 
of  Linnaeus  has  generally  been  called  sleep,  namely, 
that  of  the  petals  of  the  many  flowers  which  close  at 
night.  These  movements  have  been  ably  investigated 
by  Pfeffer,  who  has  shown  (as  was  first  observed  by 
Hofmeister)  that  they  are  caused  or  regulated  more 
by  temperature  than  by  the  alternations  of  light  and 
darkness.  Although  they  cannot  fail  to  protect  the 
organs  of  reproduction  from  radiation  at  night,  this 
does  not  seem  to  be  their  chief  function,  but  rather 
the  protection  of  the  organs  from  cold  winds,  and 
especially  from  rain,  during  the  day.  The  latter 
seems  probable,  as  Kerner  *  has  shown  that  a  widely 
different  kind  of  movement,  namely,  the  bending  down 
of  the  upper  part  of  the  peduncle,  serves  in  many 
cases  the  same  end.  The  closure  of  the  flowers  will 
also  exclude  nocturnal  insects  which  may  be  ill-adapted 
for  their-  fertilisation,  and  the  well-adapted  kinds  at 
periods  when  the  temperature  is  not  favourable  for 
fertilisation.  Whether  these  movements  of  the  petals 
consist,  as  is  probable^  of  modified  circumnutation  we 
do  not  know. 

Embryology  of  Leaves. — A  few  facts  have  been  in- 
cidentally given  in  this  chapter  on  what  may  be  called 
the  embryology  of  leaves.  With  most  plants  the 
first  leaf-  which  is  developed  after  the  cotyledons, 
resembles  closely  the  leaves  produced  by  the  mature 
plant,  but  this  is  not  always  the  case.  The  first 
leaves  produced  by  some  species  of  Drosera,  for  instance 
by  D.  Capensis,  differ  widely  in  shape  from  those 
borne  by  the  mature  plant,  and  resemble  closely  the 
leaves  of  D.  rotundifolia,  as  was  shown  to  us  by  Prof. 
Williamson  of  Manchester.  The  first  true  leaf  of 


'  Die  Schutzmittel  des  Pollens,'  1873,  pp.  30-39. 


CHAP.  VII.  FIRST-FORMED  LEAVES  415 

the  gorse,  or  Ulex,  is  not  narrow  and  spinose  like  the 
older  leaves.  On  the  other  hand,  with  many  Legumi- 
nous plants,  for  instance,  Cassia,  Acacia  lophantha,  &c., 
the  first  leaf  has  essentially  the  same  character  as  the 
older  leaves,  excepting  that  it  bears  fewer  leaflets.  In 
Trifolium  the  first  leaf  generally  bears  only  a  single 
leaflet  instead  of  three,  and  this  differs  somewhat  in 
shape  from  the  corresponding  leaflet  on  the  older  leaves. 
Now,  with  Trifolium  Pannonicum  the  first  true  leaf  on 
some  seedlings  was  unifoliate,  and  on  others  completely 
trifoliate ;  and  between  these  two  extreme  states  there 
were  all  sorts  of  gradations,  some  seedlings  bearing 
a  single  leaflet  more  or  less  deeply  notched  on  one 
or  both  sides,  and  some  bearing  a  single  additional 
and  perfect  lateral  leaflet.  Here,  then,  we  have  the 
rare  opportunity  of  seeing  a  structure  proper  to  a  more 
advanced  age,  in  the  act  of  gradually  encroaching  on 
and  replacing  an  earlier  or  embryological  condition. 

The  genus  Melilotus  is  closely  allied  to  Trifolium,  and 
the  first  leaf  bears  only  a  single  leaflet,  which  at  night 
rotates  on  its  axis  so  as  to  present  one  lateral  edge  to 
the  zenith.  Hence  it  sleeps  like  the  terminal  leaflet 
of  a  mature  plant,  as  was  observed  in  15  species,  and 
wholly  unlike  the  corresponding  leaflet  of  Trifolium, 
which  simply  bends  upwards.  It  is  therefore  a  curious 
fact  that  in  one  of  these  15  species,  viz.,  M.  Taurica  (and 
in  a  lesser  degree  in  two  others),  leaves  arising  from 
young  shoots,  produced  on  plants  which  had  been  cut 
down  and  kept  in  pots  during  the  winter  in  the  green- 
house, slept  like  the  leaves  of  a  Trifolium,  whilst  the 
leaves  on  the  fully-grown  branches  on  these  same 
plants  afterwards  slept  normally  like  those  of  a  Meli- 
lotus. If  young  shoots  rising  from  the  ground  may 
be  considered  as  new  individuals,  partaking  to  a  certain 
extent  of  the  nature  of  seedlings,  then  the  peculiar 
manner  in  which  their  leaves  slept  may  be  considered 


416  STRUCTURE   OF  CHAP.  VII. 

as  an  erubryological  habit,  probably  the  result  of  Meli- 
lotus  being  descended  from  some  form  which  slept  like 
a  Trifolium.  This  view  is  partially  supported  by  the 
leaves  on  old  and  young  branches  of  another  species, 
M.  Messanensis  (not  included  in  the  above  15  specie?), 
always  sleeping  like  those  of  a  Trifolium. 

The  first  true  leaf  of  Mimosa  albida  consists  of  a 
simple  petiole,  often  bearing  three  pairs  of  leaflets,  all 
of  which  are  of  nearly  equal  size  and  of  the  same 
shape :  the  second  leaf  differs  widely  from  the  first, 
and  resembles  that  on  a  mature  plant  (see  Fig.  159, 
p.  379),  for  it  consists  of  two  pinnae,  each  of  which 
bears  two  pairs  of  leaflets,  of  which  the  inner  basal 
one  is  very  small.  But  at  the  base  of  each  pinna 
there  is  a  pair  of  minute  points,  evidently  rudiments 
of  leaflets,  for  they  are  of  unequal  sizes,  like  the  two 
succeeding  leaflets.  These  rudiments  are  in  one  sense 
einbryological,  for  they  exist  only  during  the  youth  of 
the  leaf,  falling  off  and  disappearing  as  scfon  as  it  is 
fully  grown. 

With  Desmodium  gyrans  the  two  lateral  leaflets  are 
very  much  smaller  than  the  corresponding  leaflets  in 
most  of  the  species  in  this  large  genus ;  they  vary 
also  in  position  and  size ;  one  or  both  are  sometimes 
absent ;  and  they  do  not  sleep  like  the  fully-developed 
leaflets.  They  may  therefore  be  considered  as  almost 
rudimentary ;  and  in  accordance  with  the  general  prin- 
ciples of  embryology,  they  ought  to  be  more  constantly 
and  fully  developed  on  very  young  than  on  old  plants. 
But  this  is  not  the  case,  for  they  were  quite  absent 
on  some  young  seedlings,  and  did  not  appear  until 
from  10  to  20  leaves  had  been  formed.  This  fact 
leads  to  the  suspicion  that  D.  gyrans  is  descended 
through  a  unifoliate  form  (of  which  some  exist)  from 
a  trifoliate  species ;  and  that  the  little  lateral  leaflets 
reappear  through  reversion.  However  this  may  be, 


CHAP.  VII.  FIRST-FORMED   LEAVES.  417 

the  interesting  fact  of  the  pulvini  or  organs  of  move- 
ment of  these  little  leaflets,  not  having  been  reduced 
nearly  so  much  as  their  blades — taking  the  large 
terminal  leaflet  as  the  standard  of  comparison— gives 
us  probably  the  proximate  cause  of  their  extraordinary 
power  of  gyration. 


418  MODIFIED  CIRCUMNUTATION.         CIIAP.  VIII. 


CHAPTER  VIII. 

MODIFIED  CIRCUMNUTATION  :  MOVEMENTS  EXCITED  BY  LIGHT. 

Distinction  between  heliotropism  and  the  effects  of  light  on  the  perio- 
dicity of  the  movements  of  leaves — Heliotropic  movements  of  Beta, 
Solanum,  Zea,  and  Avena — Heliotropic  movements  towards  an 
obscure  light  in  Apios,  Brassica,  Phalaris,  Tropaeolum,  and  Cassia 
— Aplieliotropic  movements  of  tendrils  of  Bignonia — Of  flower- 
peduncles  of  Cyclamen  —  Burying  of  the  pods — Heliotropism 
and  apheliotropism  modified  forms  of  circumnutation— Steps  by 
which  one  movement  is  converted  into  the  other — Transversal- 
heliotropismus  or  diaheliotropism,  influenced  by  epinasty,  the 
•weight  of  the  part  and  apogeotropism— Apogeotropism  overcome 
during  the  middle  of  the  day  by  diaheliotropism — Effects  of  the 
weight  of  the  blades  of  cotyledons — So-called  diurnal  sleep — Chloro- 
phyll injured  by  intense  light — Movements  to  avoid  intense  light. 

SACHS  first  clearly  pointed  out  the  important  dif- 
ference between  the  action  of  light  in  modifying  the 
periodic  movements  of  leaves,  and  in  causing  them  to 
bend  towards  its  source.*  The  latter,  or  heliotropic 
movements  are  determined  by  the  direction  of  the  light, 
whilst  periodic  movements  are  affected  by  changes  in 
its  intensity  and  not  by  its  direction.  The  periodicity 
of  the  circumnutating  movement  often  continues  for 
some  time  in  darkness,  as  we  have  seen  in  the  last 
chapter ;  whilst  heliotropic  bending  ceases  very  quickly 
when  the  light  fails.  Nevertheless,  plants  which  have 
ceased  through  long-continued  darkness  to  move  pe- 
riodically, if  re-exposed  to  the  light  are  still,  according 
to  Sachs,  heliotropic. 

Apheliotropism,  or,,  as  usually  designated,  negative 

»  •  Physiologic  Veg.'  (French  Translation),  1868,  pp.  42,  517,  &c. 


CHAP.  VIII.      MOVEMENTS   EXCITED  BY  LIGHT.          419 

heliotropism,  implies  that  a  plant,  when  unequally 
illuminated  on  the  two  sides,  bends  from  the  light, 
instead  of,  as  in  the  last  sub-class  of  cases,  towards  it ; 
but  apheliotropism  is  comparatively  rare,  at  least  in  a 
well-marked  degree.  There  is  a  third  and  large  sub- 
class of  cases,  namely,  those  of  "  Transversal- Helio- 
tropismus  "  of  Frank,  which  we  will  here  call  diahelio- 
tropism.  Parts  of  plants,  under  this  influence,  place 
themselves  more  or  less  transversely  to  the  direction 
whence  the  light  proceeds,  and  are  thus  fully  illumi- 
nated. There  is  a  fourth  sub-class,  as  far  as  the  final 
cause  of  the  movement  is  concerned  ;  for  the  leaves  of 
some  plants  when  exposed  to  an  intense  and  injurious 
amount  of  light  direct  themselves,  by  rising  or  sinking 
or  twisting,  so  as  to  be  less  intensely  illuminated. 
Such  movements  have  sometimes  been  called  diurnal 
sleep.  If  thought  advisable,  they  might  be  called 
paraheliotropic,  and  this  term  would  correspond  with 
our  other  terms. 

It  will  be  shown  in  the  present  chapter  that  all  the 
movements  included  in  these  four  sub-classes,  con- 
sist of  modified  circumnutation.  We  do  not  pretend  to 
say  that  if  a  part  of  a  plant,  whilst  still  growing,  did  not 
circumnutate — though  such  a  supposition  is  most  im- 
probable— it  could  not  bend  towards  the  light ;  but,  as 
a  matter  of  fact,  heliotropism  seems  always  to  consist 
of  modified  circumnutation.  Any  kind  of  movement 
in  relation  to  light  will  obviously  be  much  facilitated 
by  each  part  circumnutating  or  bending  successively 
in  all  directions,  so  that  an  already  existing  movement 
has  only  to  be  increased  in  some  one  direction,  and  to 
be  lessened  or  stopped  in  the  other  directions,  in  order 
that  it  should  become  heliotropic,  apheliotropic,  &c., 
as  the  case  may  be.  In  the  next  chapter  some  obser- 
vations on  the  sensitiveness  of  plants  to  light,  their 


420  MODIFIED  CIRCUMNUTATION.         CHAP.  VIII. 

rate  of  bending  towards  it,  and  the  accuracy  with 
which  they  point  towards  its  source,  &c.,  will  be 
given.  Afterwards  it  will  be  shown — and  this  seems 
to  us  a  point  of  much  interest — that  sensitiveness  to 
light  is  sometimes  confined  to  a  small  part  of  the 
plant;  and  that  this  part  when  stimulated  by  light, 
transmits  an  influence  to  distant  parts,  exciting  them 
to  bend. 

Heliotropism.  —  When    a   plant   which   is   strongly 
heliotropic  (and  species  differ  much  in  this  respect) 
is  exposed  to  a  bright  lateral  light,  it  bends  quickly 
towards   it,  and   the   course  pursued  by  the  stem  is 
quite  or  nearly  straight.      But  if  the  light  is  much 
dimmed,  or  occasionally  interrupted,  or  admitted  in 
only  a  slightly  oblique  direction, 
the  course  pursued  is  more  or  less 
zigzag ;  and  as  we  have  seen  and 
shall  again  see,  such  zigzag  move- 
ment results  from  the  elongation  or 
drawing  out  of  the  ellipses,  loops, 
&c.,  which  the  plant  would  have  de- 
scribed, if  it  had  been  illuminated 
from  above.     On  several  occasions 
Beta  vegans:  circumnu-  we  were  much  struck  with  this  fact, 
tation  of  hypocotyl,  <ie-  whilst  observing  the  circumnuta- 

flected     by    the    light       .  .  ,  .    ,  ,  .   .  _.. 

being  slightly  lateral,   tion  ot  highly  sensitive  seedlings, 
traced  on  a  horizontal   which  were   unintentionally  illu- 

glass  from  8.30  A.M.  to  ITT 

5.30  P.M.  Direction  of  the  mmated  rather  obliquely,  or  only 
lighted  taper  by  which     t  sucoessive  intervals  of  time. 

it      was      illuminated, 

shown  by  a  line  joining        r      instance,  two  young  seedlings  of 

the  first  and  penultimate     _ 

dots.  Figure  reduced  to    Beta  vulgans  were  placed  in  the  middle 

one-third  of  the  original    of  a  room  with  north-east  windows,  and 

scale-  were   kept   covered  up,  except  during 

each  observation  which  lasted  for  only  a  minute  or  two ;  but  the 

result  was  that  their  hypocotyls  bowed  themselves  to  the  side, 

whence  some  light  occasionally  entered,  in  lines  which  were 


CHAP.  VIII. 


HELIOTKOPISM. 


421 


Fig.  169. 


only  slightly  zigzag.  Although  not  a  single  ellipse  was  even 
approximately  formed,  we  inferred  from  the  zigzag  lines — and, 
as  it  proved,  correctly— that  their  hypocotyls  were  circumnuta- 
ting,  for  on  the  following  day  these  same  seedlings  were  placed 
in  a  completely  darkened  room,  and  were  observed  each  time  by 
the  aid  of  a  small  wax  taper  held  almost 
directly  above  them,  and  their  movements 
were  traced  on  a  horizontal  glass  above  ; 
and  now  their  hypocotyls  clearly  circum- 
nutated  (Fig.  168,  and  Fig.  39,  formerly 
given,  p.  52);  yet  they  moved  a  short 
distance  towards  the  side  where  the  taper 
was  held  up.  If  wre  look  at  these  diagrams, 
and  suppose  that  the  taper  had  been  held 
more  on  one  side,  and  that  the  hypocotyls, 
still  circumnutating,  had  bent  themselves 
within  the  same  time  much  more  towards 
the  light,  long  zigzag  lines  would  ob- 
viously have  been  the  result. 

Again,  two  seedlings  of  Solarium  lyco- 
persicum  were  illuminated  from  above, 
but  accidentally  a  little  more  light  entered 
on  one  than  on  any  other  side,  and  their 
hypocotyls  became  slightly  bowed  towards 
the  brighter  side ;  they  moved  in  a  zigzag 
line  and-described  in  their  course  two  little 
triangles,  as  seen  in  Fig.  37  (p.  50),  and 
in  another  tracing  not  given.  The  sheath- 
like  cotyledons  of  Zea  mays  behaved,  under 
nearly  similar  circumstances,  in  a  nearly 
similar  manner,  as  described  in  our  first  ^t*™ «**».•  heliotrope 

movement  and  circum- 

chapter  (p.  64),  for  they  bowed  themselves  nutation  of  sheath-like 
during  the  whole  day  towards  one  side, 
making,  however,  in  their  course  some 
conspicuous  flexures.  Before  we  knew 
how  greatly  ordinary  circumnutation  was 
modified  by  a  lateral  light,  some  seedling  oats,  with  rather  old 
and  therefore  not  highly  sensitive  cotyledons,  were  placed  in 
front  of  a  north-east  window,  towards  which  they  bent  all  day  in 
a  strongly  zigzag  course.  On  the  following  day  they  continued 
to  bend  in  the  same  direction  (Fig.  169),  but  zigzagged  much 
The  sky,  however,  became  between  12.40  and  2.35  P.M. 


cotyledon  (1J  inch  in 
height)  traced  on  hori- 
zontal glass  from  8  A.M. 
to  10.25  P.M.  Oct.  Itith. 


422 


MODIFIED   CIRCUMNUTATION.         CHAP.  VIII. 


Fig.  170. 


•So 


overcast  with  extraordinarily  dark  thunder-clouds,  and  it  was 
interesting  to  note  how  plainly  the  cotyledons  circumnutated 
during  tliis  interval. 

The  foregoing  observations  are  of  some 
value,  from  having  been  made  when  we  were 
not  attending  to  heliotropism ;  and  they  led 
us  to  experiment  on  several  kinds  of  seed- 
lings, by  exposing  them  to  a  dim  lateral  light, 
so  as  to  observe  the  gradations  between 
ordinary  circumnutation  and  heliotropism. 
Seedlings  in  pots  were  placed  in  front  of, 
and  about  a  yard  from,  a  north-east  window ; 
on  each  side  and  over  the  pots  black  boards 
were  placed ;  in  the  rear  the  pots  were  open 
to  the  diffused  light  of  the  room,  which 
had  a  second  north-east  and  a  north-west 
window.  By  hanging  up  one  or  more  blinds 
before  the  window  where  the  seedlings  stood, 
it  was  easy  to  dim  the  light,  so  that  very 
little  more  entered  on  this  side  than  on  the 
opposite  one,  which  received  the  diifused 
light  of  the  room.  Late  in  the  evening  the 
blinds  were  successively  removed,  and  as  the 
plants  had  been  subjected  during  the  day  to 
a  very  obscure  light,  they  continued  to  bend 
towards  the  window  later  in  the  evening  than 
would  otherwise  have  occurred.  Most  of  the 
seedlings  were  selected  because  they  were 
known  to  be  highly  sensitive  to  light,  and 
some  because  they  were  but  little  sensitive, 
or  had  become  so  from  having  grown  old. 
The  movements  were  traced  in  the  usual 
manner  on  a  horizontal  glass  cover ;  a  fine 
'%  1 .1  glass  filament  with  little  triangles  of  paper 
«"  *g  having  been  cemented  in  an  upright  position 
gjf'g  to  the  hypocotyls.  Whenever  the  stem  or 
-is'g  hypocotyl  became  much  bowed  towards  the 
1  "g  -S  light,  the  latter  part  of  its  course  had  to 
135 •§  a  be  traced  on  a  vertical  glass,  parallel  to  the 
window,  and  at  right  angles  to  the  horizontal 
glass  cover. 
Apios  graveolens.—The  hypocotyl  bends  in  a  few  hours  rectan- 


CHAP.  VIII.  HELIOTKOPISM.  423 

gularly  towards  a  bright  lateral  light.  In  order  to  ascertain 
how  straight  a  course  it  would  pursue  when  fairly  well  illumi- 
nated on  one  side,  seedlings  were  first  placed  before  a  south-west 
window  on  a  cloudy  and  rainy  morning ;  and  the  movement  of 
two  hypocotyls  were  traced  for  3  h.,  during  which  time  they 
became  greatly  bowed  towards  the  light.  One  of  these  tracings 
is  given  on  p.  422  (Fig.  170),  and  the  course  may  be  seen  to  be 
almost  straight.  But  the  amount  of  lighten  this  occasion  was 
superfluous,  for  two  seedlings  were  placed  before  a  north-east 
window,  protected  by  an  ordinary  linen  and  two  muslin  blinds, 
yet  their  hypocotyls  moved  towards  this  rather  dim  light  in 
only  slightly  zigzag  lines ;  but  after  4  P.M.,  as  the  light  waned, 
the  lines  became  distinctly  zigzag.  One  of  these  seedlings, 
moreover,  described  in  the  afternoon  an  ellipse  of  considerable 
size,  with  its  longer  axis  directed  towards  the  window. 

We  now  determined  that  the  light  should  be  made  dim 
enough,  so  we  began  by  exposing  several  seedlings  before  a 
north-east  window,  protected  by  one  linen  blind,  three  muslin 
blinds,  and  a  towel.  But  so  little  light  entered  that  a  pencil 
cast  no  perceptible  shadow  on  a  white  card,  and  the  hypocotyls 
did  not  bend  at  all  towards  the  window.  During  this  time, 
from  8.15  to  10.50  A.M.,  the  hypocotyls  zigzagged  or  circum- 
nutated  near  the  same  spot,  as  may  be  seen  at  A,  in  Fig.  171 
The  towel,  therefore,  was  removed  at  10.50  A.M.,  and  replaced 
by  two  inuslin  blinds,  and  now  the  light  passed  through 
one  ordinary  linen  and  four  muslin  blinds.  When  a  pencil 
was  held  upright  on  a  card  close  to  the  seedlings,  it  cast  a 
shadow  (pointing  from  the  window)  which  could  only  just 
be  distinguished.  Yet  this  very  slight  excess  of  light  on 
one  side  sufficed  to  cause  the  hypocotyls  of  all  the  seedlings 
immediately  to  begin  bending  in  zigzag  lines  towards  the 
window,  The  course  of  one  is  shown  at  A  (Fig.  171) :  after 
moving  towards  the  window  from  10.50  A.M.  to  12.48  P.M.  it 
bent  from  the  window,  and  then  returned  in  a  nearly  parallel 
line;  that  is,  it  almost  completed  between  12.48  and  2  P.M. 
a  narrow  ellipse.  Late  in  the  evening,  as  the  light  waned, 
the  hypocotyl  ceased  to  bend  towards  the  window,  and  circum- 
nutated  on  a  small  scale  round  the  same  spot ;  during  the  night 
it  moved  considerably  backwards,  that  is,  became  more  upright, 
through  the  action  of  apogeotropism.  At  B,  we  have  a  tracing 
of  the  movements  of  another  seedling  from  the  hour  (10.50  A.M.) 
when  the  towel  was  removed ;  and  it  is  in  all  essential  respects 
19 


424 


MODIFIED   CIRCUMNUTATION.         CHAP.  VIII 


similar  to  the  previous  one.  In  these  two  cases  there  could  be 
no  doubt  that  the  ordinary  circumnutating  movement  of  the 
hypocotyl  was  modified  and  rendered  heliotropic. 


Fig.  171. 


9°fJ.tn.. 


Apios  grareolens :  heliotropic  movement  and  circumnutation  of  the  hypo- 
cotyls  of  two  seedlings  towards  a  dim  lateral  light,  traced  on  a  horizontal 
glass  during  the  day.  The  broken  lines  show  their  return  nocturnal 
courses.  Height  of  hypocotyl  of  A  -5,  and  of  B  -55  inch.  Figure  reduced 
to  one-half  of  original  scale. 

Brassica  oleracea. — The  hypocotyl  of  the  cabbage,  when  not 
disturbed  by  a  lateral  light,  circumnutates  in  a  complicated 


CHAP.  VIII.  HELIOTROPISM.  425 

manner  over  nearly  the  same  space,  and  a  figure  formerly  given 
is  here  reproduced  (Fig.  172).  If  the  hypocotyl  is  exposed  to 
a  moderately  strong  lateral  light  it  moves  quickly  towards  this 
side,  travelling  in  a  straight,  or  nearly  straight,  line.  But  when 
the  lateral  light  is  very  dim  its  course  is  extremely  tortuous,  and 
evidently  consists  of  modified  circumnutation.  Seedlings  were 
placed  before  a  north-east  window,  protected  by  a  linen  and 
muslin  blind  and  by  a  towel.  The  sky  was  cloudy,  and  when- 
ever the  clouds  grew  a  little  lighter  an  additional  muslin  blind 
was  temporarily  suspended.  The  light  from  the  window  was 

Fig.  172. 


Brassica  oleracea  :  ordinary  circumnutating  movement  of  the  hypocotyl  of 
a  seedling  plant. 

thus  so  much  obscured  that,  judging  by  the  unassisted  eye,  the 
seedlings  appeared  to  receive  more  light  from  the  interior 
of  the  room  than  from  the  window;  but  this  was  not  really 
the  case,  as  was  shown  by  a  very  faint  shadow  cast  by  a  pencil 
on  a  card.  Nevertheless,  this  extremely  small  excess  of  light 
on  one  side  caused  the  hypocotyls,  which  in  the  morning  had 
stood  upright,  to  bend  at  right  angles  towards  the  window, 
so  that  in  the  evening  (after  4.23  P.M.)  their  course  had  to  be 
traced  on  a  vertical  glass  parallel  to  the  window.  It  should  be 
stated  that  at  3.30  P.M.,  by  which  time  the  sky  had  become 
darker,  the  towel  was  removed  and  replaced  by  an  additional 
muslin  blind,  which  itself  was  removed  at  4  P.M.,  the  other  twe 


428 


MODIFIED  CIRCUMNUTATION.         CHAP.  VIII. 


blinds  being  left  suspended.    In  Fig.  173  the  course  pursued, 
between  8.9  A.M.  and  7.10  P.M.,  by  one  of  the  hypocotyls  thus 

Fig.  173. 


11: 


Brassica  oleracea  :  heliotropic  movement  and  circumnutation  of  a  hypocotyl 
towards  a  very  dim  lateral  light,  traced  during  11  hours,  on  a  horizontal 
glass  in  the  morning,  and  on  a  vertical  glass  in  the  evening.  Figure 
reduced  to  one-third  of  the  original  scale. 

exposed  is  shown.    It  may  be  observed  that  during  the  first 
16  m.  the  hypocotyl  moved  obliquely  from  the  light,  and  this, 


CHAP.  VIII. 


HELIOTEOPISM. 


427 


Fig.  174. 


no  doubt,  was  due  to  its  then  circumnutating  in  this  direction. 

Similar  cases  were  repeatedly  observed,  and  a  dim  light  rarely 

or  never  produced  any  effect  until  from  a  quarter  to  three- 
quarters  of  an  hour  had  elapsed.  After  5.15  P.M.,  by  which 

time  the  light  had  become 

obscure,     the    hypocotyl 

began    to     circumnutate 

about  the  same  spot.   The 

contrast  between  the  two 

figures     (172    and    173) 

would    have    been   more 

striking,  if  they  had  been 

originally  drawn   on  the 

same  scale,  and  had  been 

equally  reduced.    But  the 

movements  shown  in  Fig. 

1 72  were  at  first  more  mag- 
nified, and  have  been  re- 
duced to  only  one-half  of 

the  original  scale;  whereas 

those  in  Fig.  173  were  at 

first  less  magnified,  and 

have  been   reduced  to   a 

one-third  scale.  A  tracing 

made  at  the  same  time 

with    the     last    of     the 

movements  of  a    second 

hypocotyl,    presented     a 

closely  analogous  appear- 
ance ;  but  it  did  not  bend 

quite  so  much  towards  the 

light,  and  it    circumnu- 

tated  rather  more  plainly.  Solaris  Canariensis:  heliotropic  movement 
U7.  T  •  xv  •  •  '  and  circumnutation  of  a  rather  old  coty- 

Phalans  Canariensis.-  ledon?  towards  a  dull  lateral  %ht?  trace'd 

The  sheath-like  cotyledons  on  a  horizontal  glass  from  8.15  A.M.  Sept. 
of  this  monocotyledonous  16th  to  7-45  A-M-  17th-  Figure  reduced 
plant  were  selected  for  to  one-third  of  original  scale. 

trial,  because  they  are  very  sensitive  to  light  and  circumnutate 
well,  as  formerly  shown  (see  Fig.  49,  p.  63).  Although  we  felt 
no  doubt  about  the  result,  some  seedlings  were  first  placed 
before  a  south-west  window  on  a  moderately  bright  morning,  and 
the  movements  of  one  were  traced.  As  is  so  common,  it  moved 


428  MODIFIED  CIRCUMXUTATION.         CHAP.  YIIL 

for  the  first  45  m.  in  a  zigzag  line ;  it  then  felt  the  full  influence 
of  the  light,  and  travelled  towards  it  for  the  next  2h.  30  m.  in  an 
almost  straight  line.  The  tracing  has  not  been  given,  as  it  was 
almost  identical  with  that  of  Apios  under  similar  circum- 
stances (Fig.  170).  By  noon  it  had  bowed  itself  to  its  full 
extent ;  it  then  circumnutated  about  the  same  spot  and  described 
two  ellipses ;  by  5  P.M.  it  had  retreated  considerably  from  the 
light,  through  the  action  of  apogeotropism.  After  some  pre- 
liminary trials  for  ascertaining  the  right  degree  of  obscurity, 
Forne  seedlings  were  placed  (Sept.  16th)  before  a  north-east 
window,  and  light  was  admitted  through  an  ordinary  linen 
and  three  muslin  blinds.  A  pencil  held  close  by  the  pot  now 
cast  a  very  faint  shadow  on  a  white  card,  pointing  from  the 
window.  In  the  evening,  at  4.30,  and  again  at  6  P.M.,  some  of 
the  blinds  were  removed.  In  Fig.  174  we  see  the  course  pursued 
under  these  circumstances  by  a  rather  old  and  not  very  sensitive 
cotyledon,  1'9  inch  in  height,  which  became  much  bowed, 
but  was  never  rectangularly  bent  towards  the  light.  From 
11  A.M.,  when  the  sky  became  rather  duller,  until  6.30  P.M.,  the 
zigzagging  was  conspicuous,  and  evidently  consisted  of  drawn- 
out  ellipses.  After  6.30  P.M.  and  during  the  night,  it  retreated 
in  a  crooked  line  from  the  window.  Another  and  younger  seed- 
ling moved  during  the  same  time  much  more  quickly  and  to  a 
much  greater  distance,  in  an  only  slightly  zigzag  line  towards 
the  light ;  by  11  A.M.  it  was  bent  almost  rectangularly  in  this 
direction,  and  now  circumnutated  about  the  same  place. 

Tropceolum  majus. — Some  very  young  seedlings,  bearing  only 
two  leaves,  and  therefore  not  as  yet  arrived  at  the  climbing 
stage  of  growth,  were  first  tried  before  a  north-east  window 
without  any  blind.  The  epicotyls  bowed  themselves  towards 
the  light  so  rapidly  that  in  little  more  than  3  h.  their  tips 
pointed  rectangularly  towards  it.  The  lines  traced  were  either 
nearly  straight  or  slightly  zigzag ;  and  in  this  latter  case  we 
see  that  a  trace  of  circumnutation  was  retained  even  under  the 
influence  of  a  moderately  bright  light.  Twice  whilst  these 
epicotyls  were  bending  towards  the  window,  dots  were  made 
every  5  or  6  minutes,  in  order  to  detect  any  trace  of  lateral 
movement,  but  there  was  hardly  any ;  and  the  lines  formed  by 
their  junction  were  nearly  straight,  or  only  very  slightly  zigzag, 
as  in  the  other  parts  of  the  figures.  After  the  epicotyls  had 
bowed  themselves  to  the  full  extent  towards  the  light,  ellipses 
of  considerable  size  were  described  in  the  usual  manner. 


CHAP.  VIII.  HELIOTROPISM.  429 

After  having  seen  hove  the  epicotyls  moved  towards  a  mode- 
rately bright  light,  seedlings  were  placed  at  7.48  A.M.  (Sept.  7th) 
before  a  north-east  window,  covered  by  a  towel,  and  shortly 
afterwards  by  an  ordinary  linen  blind,  but  the  epicotyls  still 
moved  towards  the  window.  At  9.13  A.M.  two  additional  muslin 
blinds  were  suspended,  so  that  the  seedlings  received  very  little 
more  light  from  the  window  than  from  the  interior  of  the  room. 
The  sky  varied  in  brightness,  and  the  seedlings  occasionally 

Fig.  175. 


Tropaolum  majus :  heliotropic  movement  and  circumnutation  of  the  epicotyl 
of  a  young  seedling  towards  a  dull  lateral  light,  traced  on  a  horizontal 
glass  from  7.48  A.M.  to  10.40  P.M.  Figure  reduced  to  one-half  of  the 
original  scale. 

received  for  a  short  time  less  light  from  the  window  than  from 
the  opposite  side  (as  ascertained  by  the  shadow  cast),  and  then 
one  of  the  blinds  was  temporarily  removed.  In  the  evening 
the  blinds  were  taken  away,  one  by  one.  The  course  pursued 
by  an  epicotyl  under  these  circumstances  is  shown  in  Fig.  175. 
During  the  whole  day,  until  6.45  P  M.,  it  plainly  bowed  itself 
towards  the  light ;  and  the  tip  moved  over  a  considerable  space. 
After  6.45  P.M.  it  moved  backwards,  or  from  the  window,  till 


430 


MODIFIED   CIRCUMNUTATION. 


CHAP.  VIII. 


Fig.  176. 


10.40  P.M.,  when  the  last  dot  was  made.  Here,  then,  we  have 
a  distinct  heliotropic  movement,  effected  by  means  of  six 
elongated  figures  (which  if  dots  had  been  made  every  few 
minutes  would  have  been  more  or  less  elliptic)  directed  towards 

the  light,  with  the  apex  of  each  suc- 
cessive ellipse  nearer  to  the  window 
than  the  previous  one.  Now,  if  the 
light  had  been  only  a  little  brighter, 
the  epicotyl  would  have  bowed  itself 
more  to  the  light,  as  we  may  safely 
conclude  from  the  previous  trials ; 
there  would  also  have  been  less 
lateral  movement,  and  the  ellipses  or 
other  figures  would  have  been  drawn 
out  into  a  strongly  marked  zigzag 
line,  with  probably  one  or  two  small 
loops  still  formed.  If  the  light  had 
been  much  brighter,  we  should  have 
had  a  slightly  zigzag  line,  or  one 
quite  straight,  for  there  would  have 
been  more  movement  in  the  direc- 
tion of  the  light,  and  much  less  from 
side  to  side. 

Sachs  states  that  the  older  inter - 
nodes  of  this  Tropseolum  are  aphe- 
liotopic;  we  therefore  placed  a 
plant,  111  inches  high,  in  a  box, 
blackened  within,  but  open  on  one 
side  in  front  of  a  north-east  window 
without  any  blind.  A  filament  was 
fixed  to  the  third  internode  from 
the  summit  on  one  plant,  and  to 
the  fourth  internode  of  another. 
These  internodes  were  either  not 
old  enough,  or  the  light  was  not  suf- 
ficiently bright,  to  induce  aphelio- 
tropism,  for  both  plants  bent  slowly  towards,  instead  of  from 
the  window  during  four  days.  The  course,  during  two  days  of 
the  first-mentioned  internode,  is  given  in  Fig.  176  ;  and  we  see 
that  it  either  circumnutated  on  a  small  scale,  or  travelled  in  a 
zigzag  line  towards  the  light.  We  have  thought  this  case  of 
feeble  heliotropism  in  one  of  the  older  internodes  of  a  plant, 


Tropaeolum,  majus :  heliotropic 
movement  and  circumnuta- 
tion  of  an  old  internode  to- 
wards a  lateral  light,  traced 
on  a  horizontal  glass  from  8 
A.M.  Nov.  2nd  to  10.20  A.M. 
Nov.  4th.  Broken  lines  show 
the  nocturnal  course. 


CHAP.  VIII. 


HELIOTROPISM. 


431 


Fig.  177. 


which,  whilst  young,  is  so  extremely  sensitive  to  light,  woith 
giving. 

Cassia  tora.  —  The  cotyledons  of  this  plant  are  extremely 
sensitive  to  light,  whilst  the 
hypocotyls  are  much  less 
sensitive  than  those  of  most 
other  seedlings,  as  we  had 
often  observed  with  surprise. 
It  seemed  therefore  worth 
while  to  trace  their  move- 
ments. They  were  exposed 
to  a  lateral  light  before  a 
north-east  window,  which 
was  at  first  covered  merely 
by  a  muslin  blind,  but  as 
the  sky  grew  brighter  about 
11  A.M.,  an  additional  linen 
blind  was  suspended.  After 
4  P.M.  one  blind  and  then  the 
other  was  removed.  The 
seedlings  were  protected  on 
each  side  and  above,  but  were 
open  to  the  diffused  light 
of  the  room  in  the  rear.  Up- 
right filaments  were  fixed  to 
the  hypocotyls  of  two  seed- 
lings, which  stood  vertically 
in  the  morning.  The  accom- 
panying figure  (Fig.  177) 
shows  the  course  pursued  by 
one  of  them  during  two  days ; 
but  it  should  be  particularly 
noticed  that  during  the 
second  day  the  seedlings  were 

kept  in  darkness,  and  they    Cas*ia  tora  -\  heliotropic  movement  and 
then    circumnutated    round 
nearly  the  same  small  space. 
On  the  first  day  (Oct.  7th) 
the  hypocotyl    moved  from 
8  A.M.  to  12.23  P.M.,  toward 
the  light  in  a  zigzag  line,  then  turned  abruptly  to  the  left 
and  afterwards  described  a  small   ellipse.     Another  irregular 


circumnutation  of  a  hypocotyl. 
inch  in  height)  traced  on  a  horizontal 
glass  from  8  A.M.  to  10.10  P.M.  Oct. 
7th.  Also  its  circumnutation  in 
darkness  from  7  A.M.  Oct.  8th  to  7.45 
A.M  Oct.  9th. 


432 


MODIFIED   CIRCUMNUTATION.        CHAP.  VIII. 


ellipse    was    completed    between   3   P.M.  and  about  5.30  P.M., 
the  hypocotyl  still  bending  towards  the  light.     The  hypocotyl 


Fig.  178. 


was  straight  and  upright  in  the  morn- 
ing, but  by  6  P.M.  its  upper  half  was 
bowed  towards  the  light,  so  that  the 
chord  of  the  arc  thus  formed  stood  at 
an  angle  of  20°  with  the  perpendicular. 
After  6  P.M.  its  course  was  reversed 
through  the  action  of  apogeotropism. 
and  it  continued  to  bend  from  the 
window  during  the  night,  as  shown  by 
the  broken  line.  On  the  next  day  it 
was  kept  in  the  dark  (excepting  when 
each  observation  was  made  by  the  aid 
of  a  taper),  and  the  course  followed 
from  7  A.M.  on  the  8th  to  7.45  A.M.  on 
the  9th  is  here  likewise  shown.  The 
difference  between  the  two  parts  of  the 
figure  (177),  namely,  that  described 
during  the  daytime  on  the  7th,  when 
exposed  to  a  rather  dim  lateral  light, 
and  that  on  the  8th  in  darkness,  is 
striking.  The  difference  consists  in  the 
lines  during  the  first  day  having  been 
drawn  out  in  the  direction  of  the  light. 
The  movements  of  the  other  seedling, 
traced  under  the  same  circumstances, 
were  closely  similar. 

Apheliotropism. — We  succeeded  in 
observing  only  two  cases  of  aphelio- 
tropism,  for  these  are  somewhat  rare ; 
and  the  movements  are  generally  so 
slow  that  they  would  have  been  very 
troublesome  to  trace. 

Bignonia  capreolata. — No  organ  of 
any  plant,  as  far  as  we  have  seen,  bends 
away  so  quickly  from  the  light  as  do 
the  tendrils  of  this  Bignonia.  They 
are  also  remarkable  from  circum- 
nutating  much  less  regularly  than 
most  other  tendrils,  often  remaining 
stationary;  they  depend  on  apheliotropism  for  coming  into 


Bignonia  capreolata :  aphe- 
liotropic  movement  of  a 
tendril,  traced  on  a  hoi-i- 
zontal  glass  from  6.45 
A.M.  July  19th  to  10  A.M. 
20th.  Movements  as 
originally  traced,  little 
magnified,  here  reduced 
to  two-thirds  of  the 
original  scale. 


CHAP.  VIII.  APHELIOTROPISM.  433 

contact  with  the  trunks  of  trees.*  The  stem  of  a  young  plant 
was  tied  to  a  stick  at  the  base  of  a  pair  of  fine  tendrils,  which 
projected  almost  vertically  upwards;  and  it  was  placed  in 
front  of  a  north-east  window,  being  protected  on  all  other  sides 
from  the  light.  The  first  dot  was  made  at  6.45  A.M.,  and  by 
7.35  A.M.  both  tendrils  felt  the  full  influence  of  the  light,  for 
they  moved  straight  away  from  it  until  9.20  A.M.,  when  they 
circumnutated  for  a  time,  still  moving,  but  only  a  little,  from 
the  light  (see^Fig.  178  of  the  left-hand  tendril).  After  3  P.M. 
they  again  moved  rapidly  away  from  the  light  in  zigzag  lines. 
By  a  late  hour  in  the  evening  both  had  moved  so  far,  that 
they  pointed  in  a  direct  line  from  the  light.  During  the  night 
they  returned  a  little  in  a  nearly  opposite  direction.  On  the 
following  morning  they  again  moved  from  the  light  and  con- 
verged, so  that  by  the  evening  they  had  become  interlocked, 
still  pointing  from  the  light.  The  right-hand  tendril,  whilst 
converging,  zigzagged  much  more  than  the  one  figured.  Both 
tracings  showed  that  the  apheliotropic  movement  was  a  modi- 
fied form  of  circumnutation. 

Cyclamen  Persicum. — Whilst  this  plantis  in  flower  the  peduncles 
stand  upright,  but  their  uppermost  part  is  hooked  so  that  the' 
flower  itself  hangs  downwards.  As  soon  as  the  pods  begin  to 
swell,  the  peduncles  increase  much  in  length  and  slowly  curve 
downwards,  but  the  short,  upper,  hooked  part  straightens  itself. 
Ultimately  the  pods  reach  the  ground,  and  if  this  is  covered 
with  moss  or  dead  leaves,  they  bury  themselves.  We  have  often 
seen  saucer-like  depressions  formed  by  the  pods  in  damp  sand 
or  sawdust ;  and  one  pod  ( •  3  of  inch  in  diameter)  buried  itself 
in  sawdust  for  three-quarters  of  its  length. f  We  shall  have 
occasion  hereafter  to  consider  the  object  gained  by  this  burying 
process.  The  peduncles  can  change  the  direction  of  their  cur- 
vature, for  if  a  pot,  with  plants  having  their  peduncles  already 
bowed  downwards,  be  placed  horizontally,  tjiey  slowly  bend 
at  right  angles  to  their  former  direction  towards  the  centre  of 
the  earth.  We  therefore  at  first  attributed  the  movement  to 
geotropism ;  but  a  pot  which  had  lain  horizontally  with  the  pods 


*  'The  Movements  and  Habits  tanic  Garden,'  Canto.,  iii.  p.  126), 

of  Climbing  Plants,'  1875,  p.  97.  the  pods   forcibly  penetrate  the 

*    f  The    peduncles    of    several  earth.      See     also    Grenier    and 

other  species  of  Cyclamen   twist  Godron, 'Florede  France,' torn.  ii. 

themselves  into  a  spire,  and  ac-  p.  459. 
cording  to  Erasmus  Darwin  ('  Bo- 


434  MODIFIED  CIKCTJMNUTATION.         CHAP.  VIII. 

all  pointing  to  the  ground,  was  reversed,  being  still  kept  hori- 
zontal, so  that  the  pods  now  pointed  directly  upwards ;  it  was 
then  placed  in  a  dark  cupboard,  but  the  pods  still  pointed  up- 
wards after  four  days  and  nights.  The  pot,  in  the  same  position, 
was  next  brought  back  into  the  light,  and  after  two  days  there 
was  some  bending  downwards  of  the  peduncles,  and  on  the  fourth 
day  two  of  them  pointed  to  the  centre  of  the  earth,  as  did  the 
others  after  an  additional  day  or  two.  Another  plant,  in  a  pot 
which  had  always  stood  upright,  was  left  in  the  dark  cupboard 
for  six  days ;  it  bore  3  peduncles,  and  only  one  became  within  this 

Fig.  179. 


Cyclamen  Persicum:  downward  apheliotropic  movement  of  a  flower-peduncle, 
greatly  magnified  (about  47  times  ?),  traced  on  a  horizontal  glass  from 
1  P.M.  Feb.  18th  to  8  A.M.  21st. 

time  at  all  bowed  Downwards,  and  that  doubtfully.  The  weight, 
therefore,  of  the  pods  is  not  the  cause  of  the  bending  down. 
This  pot  was  then  brought  back  into  the  light,  and  after  three 
days  the  peduncles  were  considerably  bowed  downwards.  We 
are  thus  Jed  to  infer  that  the  downward  curvature  is  due  to 
apheliotropism ;  though  more  trials  ought  to  have  been  made. 

In  order  to  observe  the  nature  of  this  movement,  a  peduncle 
bearing  a  large  pod  which  had  reached  and  rested  on  the 
ground,  was  lifted  a  little  up  and  secured  to  a  stick.  A  filament 
was  fixed  across  the  pod  with  a  mark  beneath,  and  its  move- 


CHAP.  VIII.  APHELIOTKOPISM.  435 

ment,  greatly  magnified,  was  traced  on  a  horizontal  glass  during 
.67  h.  The  plant  was  illuminated  during  the  day  from  above.  A 
copy  of  the  tracing  is  given  on  p.  434  (Fig.  179) ;  and  there  can 
be  no  doubt  that  the  descending  movement  is  one  of  modified 
circumnutation,  but  on  an  extremely  small  scale.  The  observa- 
tion was  repeated  on  another  pod,  which  had  partially  buried 
itself  in  sawdust,  and  which  was  lifted  up  a  quarter  of  an  inch 
above  the  surface ;  it  described  three  very  small  circles  in  24  h. 
Considering  the  great  length  and  thinness  of  the  peduncles 
and  the  lightness  of  the  pods,  we  may  conclude  that  they 
would  not  be  able  to  excavate  saucer-like  depressions  in  sand 
or  sawdust,  or  bury  themselves  in  moss,  &c.,  unless  they  were 
aided  by  their  continued  rocking  or  circumnutating  move- 
ment. 

Relation  between  Circumnutation  and  Heliotropism. — 
Any  one  who  will  look  at  the  foregoing  diagrams, 
showing  the  movements  of  the  stems  of  various  plants 
towards  a  lateral  and  more  or  less  dimmed  light,  will 
be  forced  to  admit  that  ordinary  circumnutation  and 
heliotropism  graduate  into  one  another.  When  a 
plant  is  exposed  to  a  dim  lateral  light  and  continues 
during  the  whole  day  bending  towards  it,  receding 
late  in  the  evening,  the  movement  unquestionably  is 
one  of  heliotropism.  Now,  in  the  case  of  Tropaeolum 
(Fig.  175)  the  stem  or  epicotyl  obviously  ciicumnu- 
tated  during  the  whole  day,  and  yet  it  continued  at 
the  same  time  to  move  heliotropically ;  this  latter 
movement  being  effected  by  the  apex  of  each  succes- 
sive elongated  figure  or  ellipse  standing  nearer  to 
the  light  than  the  previous  one.  In  the  case  of 
Cassia  (Fig.  177)  the  comparison  of  the  movement  of 
the  hypocotyl,  when  exposed  to  a  dim  lateral  light  and 
to  darkness,  is  very  instructive ;  as  is  that  between 
the  ordinary  circumnutating  movement  of  a  seedling 
Brassica  (Figs.  172,  173),  or  that  of  Phalaris  (Figs. 
49,  174),  and  their  heliotropic  movement  towards  a 
window  protected  by  blinds.  In  both  these  cases, 


436  RELATION   BETWEEN  CHAP.  VIII. 

and  in  many  others,  it  was  interesting  to  notice  bow 
gradually  the  stems  began  to  cireumnutate  as  the 
light  waned  in  the  evening.  We  have  therefore  many 
kinds  of  gradations  from  a  movement  towards  the  light, 
which  must  be  considered  as  one  of  circurnnutation 
very  slightly  modified  and  still  consisting  of  ellipses 
or  circles, — though  a  movement  more  or  less  strongly 
zigzag,  with  loops  or  ellipses  occasionally  formed, — to 
a  nearly  straight,  or  even  quite  straight,  heliotropic 
course. 

A  plant,  when  exposed  to  a  lateral  light,  though 
this  may  be  bright,  commonly  moves  at  first  in  a 
zigzag  line,  or  even  directly  from  the  light ;  and 
this  no  doubt  is  due  to  its  circumnutating  at  the 
time  in  a  direction  either  opposite  to  the  source  of 
the  light,  or  more  or  less  transversely  to  it.  As  soon, 
however,  as  the  direction  of  the  circumnutating  move- 
ment nearly  coincides  with  that  of  the  entering  light, 
the  plant  bends  in  a  straight  course  towards  the  light, 
if  this  is  bright.  The  course  appears  to  be  rendered 
more  and  more  rapid  and  rectilinear,  in  accordance  with 
the  degree  of  brightness  of  the  light — firstly,  by  the 
longer  axes  of  the  elliptical  figures,  which  the  plant 
continues  to  describe  as  long  as  the  light  remains  very 
dim,  being  directed  more  or  less  accurately  towards 
its  source,  and  by  each  successive  ellipse  being  de- 
scribed nearer  to  the  light.  Secondly,  if  the  light 
is  only  somewhat  dimmed,  by  the  acceleration  and 
increase  of  the  movement  towards  it,  and  by  the 
retardation  or  arrestment  of  that  from  the  light,  some 
lateral  movement  being  still  retained,  for  the  light 
will  interfere  less  with  a  movement  at  right  angles 
to  its  direction,  than  with  one  in  its  own  direction.* 


*  In  his  paper,  *  IT t:ber  ortho-       theile*  (*  Arbeiten  des  Bot.  Inst. 
trope  und  plagiotrope  Pflanzeu-       in  "Wiirzburg/  Band  ii.  Heft  ii. 


CHAP.  VIII.  CIBCUMNUTATION  AND  HELIOTKOPISM.    437 

The  result  is  that  the  course  is  rendered  more  or  less 
zigzag  arid  unequal  in  rate.  Lastly,  when  the  light 
is  very  bright  all  lateral  movement  is  lost ;  and  the 
whole  energy  of  the  plant  is  expended  in  rendering 
the  circumnutating  movement  rectilinear  and  rapid  in 
one  direction  alone,  namely,  towards  the  light. 

The  common  view  seems  to  be  that  heliotropism  is 
a  quite  distinct  kind  of  movement  from  circumnuta- 
tion ;  and  it  may  be  urged  that  in  the  foregoing 
diagrams  we  see  heliotropism  merely  combined  with, 
or  superimposed  on,  circumnutation.  But  if  so,  it  must 
be  assumed  that  a  bright  lateral  light  completely 
stops  circumnutation,  for  a  plant  thus  exposed  moves 
in  a  straight  line  towards  it,  without  describing  any 
ellipses  or  circles.  If  the  light  be  somewhat  obscured, 
though  amply  sufficient  to  cause  the  plant  to  bend 
towards  it,  we  have  more  or  less  plain  evidence  of  still- 
continued  circumnutation.  It  must  further  be  assumed 
that  it  is  only  a  lateral  light  which  has  this  extraor- 
dinary power  of  stopping  circumnutation,  for  we  know 
that  the  several  plants  above  experimented  on,  and 
all  the  others  which  were  observed  by  us  whilst  grow- 
ing, continue  to  circumnutate,  however  bright  the  light 
may  be,  if  it  comes  from  above.  Nor  should  it  be 
forgotten  that  in  the  life  of  each  plant,  circumnuta- 
tion precedes  heliotropism,  for  hypocotyls,  epicotyls, 
and  petioles  circumnutate  before  they  have  broken 
through  the  ground  and  have  ever  felt  the  influence  of 
light. 

We  are  therefore  fully  justified,  as  it  seems  to  us,  in 
believing  that  whenever  light  enters  laterally,  it  is  the 


1879),  Sachs  has  discussed    the  the  organs  of  plants  stand  with 

manner  in  which  geotropism  and  respect  to    the  direction  of  the 

heliotropisni  are  affected  by  dif-  incident  force, 
ferences   in  the  angles  at  which 


438  MODIFIED  CIRCUMNUTATION.         CHAP.  VIII. 

movement  of  circumnutation  which  gives  rise  to,  or  is 
converted  into,  heliotropism  and  apheliotropism.  On 
this  view  we  need  not  assume  against  all  analogy  that 
a  lateral  light  entirely  stops  circumnutation  ;  it  merely 
excites  the  plant  to  modify  its  movement  for  a  time 
in  a  beneficial  manner.  The  existence  of  every  pos- 
sible gradation,  between  a  straight  course  towards  a 
lateral  light  and  a  course  consisting  of  a  series  of  loops 
or  ellipses,  becomes  perfectly  intelligible.  Finally, 
the  conversion  of  circumnutation  into  heliotropism  or 
apheliotropism,  is  closely  analogous  to  what  takes  place 
with  sleeping  plants,  which  during  the  daytime  de- 
scribe one  or  more  ellipses,  often  moving  in  zigzag  lines 
and  making  little  loops ;  for  when  they  begin  in  the 
evening  to  go  to  sleep,  they  likewise  expend  all  their 
energy  in  rendering  their  course  rectilinear  and  rapid. 
In  the  case  of  sleep-movements,  the  exciting  or  regu- 
lating cause  is  a  difference  in  the  intensity  of  the 
light,  coming  from  above,  at  different  periods  of  the 
twenty-four  hours ;  whilst  with  heliotropic  and  aphe- 
liotropic  movements,  it  is  a  difference  in  the  intensity 
of  the  light  on  the  two  sides  of  the  plant/ 

Transvcrsal-heliotropismus  (of  Frank  *)  or  DialieUo- 
tropism. — The  cause  of  leaves  placing  themselves 
more  or  less  transversely  to  the  light,  with  their 
upper  surfaces  directed  towards  it,  has  been  of  late 
the  subject  of  much  controversy.  We  do  not  here 
refer  to  the  object  of  the  movement,  which  no  doubt 
is  that  their  upper  surfaces  may  be  fully  illuminated, 
but  the  means  by  which  this  position  is  gained. 
Hardly  a  better  or  more  simple  instance  can  be  given 


*  'Die  naturliehe  Wagerechte  Frage  iiber  Transversal  Geo-und 

Riehtung    von    Pflanzeritheilen,'  Heliotropismus/'  '  Bot.  Zeitung,' 

18' 70.     See  also  some  interesting  1873,  p.  17  et  seq. 
articles  by  the  same  author,  "  Zur 


CHAP.  VIII.  DIAHELIOTROPISM.  439 

of  diaheliotropism  than  that  offered  by  many  seed- 
lings, the  cotyledons  of  which  are  extended  hori- 
zontally. When  they  first  burst  from  their  seed-coats 
they  are  in  contact  and  stand  in  various  positions, 
often  vertically  upwards ;  they  soon  diverge,  and  this 
is  effected  by  epinasty,  which,  as  we  have  seen,  is  a 
modified  form  of  circunmutation.  After  they  have 
diverged  to  their  full  extent,  they  retain  nearly  the 
same  position,  though  brightly  illuminated  all  day 
long  from  above,  with  their  lower  surfaces  close  to  the 
ground  and  thus  much  shaded.  There  is  therefore  a 
great  contrast  in  the  degree  of  illumination  of  their 
upper  and  lower  surfaces,  and  if  they  were  heliotropic 
they  would  bend  quickly  upwards.  It  must  not,  how- 
ever, be  supposed  that  such  cotyledons  are  immovably 
fixed  in  a  horizontal  position.  When  seedlings  are 
exposed  before  a  window,  their  hypocotyls,  which  are 
highly  heliotropic,  bend  quickly  towards  it,  and  the 
upper  surfaces  of  their  cotyledons  still  remain  ex- 
posed at  right  angles  to  the  light ;  but  if  the  hypo- 
cotyl  is  secured  so  that  it  cannot  bend,  the  cotyledons 
themselves  change  their  position.  If  the  two  are 
placed  in  the  line  of  the  entering  light,  the  one 
furthest  from  it  rises  up  and  that  nearest  to  it  often 
sinks  down ;  if  placed  transversely  to  the  light,  they 
twist  a  little  laterally;  so  that  in  every  case  they 
endeavour  to  place  their  upper  surfaces  at  right  angles 
to  the  light.  So  it  notoriously  is  with  the  leaves  on 
plants  nailed  against  a  wall,  or  grown  in  front  of  a 
window.  A  moderate  amount  of  light  suffices  to  in- 
duce such  movements ;  all  that  is  necessary  is  that  the 
light  should  steadily  strike  the  plants  in  an  oblique 
direction.  With  respect  to  the  above  twisting  move- 
ment of  cotyledons,  Frank  has  given  many  and  much 
more  striking  instances  in  the  case  of  the  leaves  on 


440  MODIFIED  CIECUMXUTATION.         CHAP. 

branches  which  had  been  fastened  in  various  positions 
or  turned  upside  down. 

In  our  observations  on  the  cotyledons  of  seedling 
plants,  we  often  felt  surprise  at  their  persistent  hori- 
zontal position  during  the  day,  and  were  convinced 
before  we  had  read  Frank's  essay,  that  some  special 
explanation  was  necessary.  De  Yries  has  shown* 
that  the  more  or  less  horizontal  position  of  leaves  is 
in  most  cases  influenced  by  epinasty,  by  their  own 
weight,  and  by  apogeotropism.  A  young  cotyledon 
or  leaf  after  bursting  free  is  brought  down  into  its 
proper  position,  as  already  remarked,  by  epinasty, 
which,  according  to  De  Yries,  long  continues  to  act 
on  the  midribs  and  petioles.  Weight  can  hardly  be 
influential  in  the  case  of  cotyledons,  except  in  a  few 
cases  presently  to  be  mentioned,  but  must  be  so  with 
large  and  thick  leaves.  With  respect  to  apogeotropism, 
De  Vries  maintains  that  it  generally  comes  into  play, 
and  of  this  fact  we  shall  presently  advance  some 
indirect  evidence.  But  over  these  and  other  constant 
forces  we  believe  that  there  is  in  many  cases,  but  we 
do  not  say  in  all,  a  preponderant  tendency  in  leaves 
and  cotyledons  to  place  themselves  more  or  less  trans- 
versely with  respect  to  the  light. 

In  the  cases  above  alluded  to  of  seedlings  exposed 
to  a  lateral  light  with  their  hypocotyls  secured,  it  is 
impossible  that  epinasty,  weight  and  apogeotropism, 
either  in  opposition  or  combined,  can  be  the  cause  of 
the  rising  of  one  cotyledon,  and  of  the  sinking  of  the 
other,  since  the  forces  in  question  act  equally  on  both  ; 
and  since  epinasty,  weight  and  apogeotropism  all  act 
in  a  vertical  plane,  they  cannot  cause  the  twisting  of 
the  petioles,  which  occurs  in  seedlings  under  the 


*  « Arbeilen  des  Bot.  Instituts  in  Wurzburg,'  Heft.  ii.  1872,  pp. 
223-277. 


CHAP.  VIII.  DIAHELIOTROPISM.  441 

above  conditions  of  illumination.  All  these  movements 
evidently  depend  in  some  manner  on  the  obliquity  of 
the  light,  but  cannot  be  called  heliotropic,  as  this 
implies  bending  towards  the  light ;  whereas  the  coty- 
ledon nearest  to  the  light  bends  in  an  opposed  direc- 
tion or  downwards,  and  both  place  themselves  as  nearly 
as  possible  at  right  angles  to  the  light.  The  move- 
ment, therefore,  deserves  a  distinct  name.  As  coty- 
ledons and  leaves  are  continually  oscillating  up  and 
down,  and  yet  retain  all  day  long  their  proper  position 
with  their  upper  surfaces  directed  transversely  to  the 
light,  and  if  displaced  reassume  this  position,  dia- 
heliotropism  must  be  considered  as  a  modified  form  of 
circumnutation.  This  was  often  evident  when  the 
movements  of  cotyledons  standing  in  front  of  a  window 
were  traced.  We  see  something  analogous  in  the  case 
of  sleeping  leaves  or  cotyledons,  which  after  oscillating 
up  and  down  during  the  whole  day,  rise  into  a  vertical 
position  late  in  the  evening,  and  on  the  following 
morning  sink  down  again  into  their  horizontal  or  dia- 
heliotropic  position,  in  direct  opposition  to  heliotro- 
pism.  This  return  into  their  diurnal  position,  which 
often  requires  an  angular  movement  of  90°,  is  analo- 
gous to  the  movement  of  leaves  on  displaced  branches, 
which  recover  their  former  positions.  It  deserves 
notice  that  any  force  such  as  apogeotropism,  will  act 
with  different  degrees  of  power*  in  the  different  posi- 
tions of  those  leaves  or  cotyledons  which  oscillate 
largely  up  and  down  during  the  day;  and  yet  they 
recover  their  horizontal  or  diaheliotropic  position. 

We  may  therefore  conclude  that  diaheliotropic 
movements  cannot  be  fully  explained  by  the  direct 
action  of  light,  gravitation,  weight,  &c.,  any  more 


*  See  former  note,  in  reference  to  Sachs'  remarks  on  this  subject. 


442  MODIFIED  CIRCUMNUTATION.         CHAP.  VHI. 

than  can  the  nyctitropic  movements  of  cotyledons 
and  leaves.  In  the  latter  case  they  place  themselves 
so  that  their  upper  surfaces  may  radiate  at  night 
as  little  as  possible  into  open  space,  with  the  upper 
surfaces  of  the  opposite  leaflets  often  in  contact.  These 
movements,  which  are  sometimes  extremely  complex, 
are  regulated,  though  not  directly  caused,  by  the  alter- 
nations of  light  and  darkness.  In  the  case  of  diahelio- 
tropism,  cotyledons  and  leaves,  place  themselves  so 
that  their  upper  surfaces  may  be  exposed  to  the  light, 
and  this  movement  is  regulated,  though  not  directly 
caused,  by  the  direction  whence  the  light  proceeds.  In 
both  cases  the  movement  consists  of  circumnutation 
modified  by  innate  or  constitutional  causes,  in  the 
same  manner  as  with  climbing  plants,  the  circumnu- 
tation of  which  is  increased  in  amplitude  and  rendered 
more  circular,  or  again  with  very  young  cotyledons 
and  leaves  which  are  thus  brought  down  into  a  hori- 
zontal position  by  epinasty. 

We  have  hitherto  referred  only  to  those  leaves  and 
cotyledons  which  occupy  a  permanently  horizontal 
position ;  but  many  stand  more  or  less  obliquely,  and 
some  few  upright.  The  cause  of  these  differences  of 
position  is  not  known ;  but  in  accordance  with  Wiesner's 
views,  hereafter  to  be  given,  it  %is  probable  that  some 
leaves  and  cotyledons  would  suffer,  if  they  were  fully 
illuminated  by  standing  at  right  angles  to  the  light. 

We  have  seen  in  the  second  and  fourth  chapters 
that  those  cotyledons  and  leaves  which  do  not  alter 
their  positions  at  night  sufficiently  to  be  said  to  sleep, 
commonly  rise  a  little  in  the  evening  and  fall  again 
on  the  next  morning,  so  that  they  stand  during  the 
night  at  a  rather  higher  inclination  than  during  the 
middle  of  the  day.  It  is  incredible  that  a  rising 
movement  of  2°  or  3°,  or  even  of  10°  or  20°,  can  be  of 


CHAP.  VIII.  DIAHELIOTEOPISM.  443 

any  service  to  the  plant,  so  as  to  have  been  specially 
acquired.  It  must  be  the  result  of  some  periodical 
change  in  the  conditions  to  which  they  are  subjected, 
and  there  can  hardly  be  a  doubt  that  this  is  the  daily 
alternations  of  light  and  darkness.  De  Yries  states  in 
the  paper  before  referred  to,  that  most  petioles  and 
midribs  are  apogeotropic  ;*  and  apogeotropism  would 
account  for  the  above  rising  movement,  which  is  com- 
mon to  so  many  widely  distinct  species,  if  we  suppose  it 
to  be  conquered  by  diaheliotropism  during  the  middle 
of  the  day,  as  long  as  it  is  of  importance  to  the  plant 
that  its  cotyledons  and  leaves  should  be  fully  exposed 
to  the  light.  The  exact  hour  in  the  afternoon  at  which 
they  begin  to  bend  slightly  upwards,  and  the  extent  of 
the  movement,  will  depend  on  their  degree  of  sen- 
sitiveness to  gravitation  and  on  their  power  of  resist- 
ing its  -action  during  the  middle  of  the  day,  as  well  as 
on  the  amplitude  of  their  ordinary  circumnutating 
movements ;  and  as  these  qualities  differ  much  in  dif- 
ferent species,  we  might  expect  that  the  hour  in  the 
afternoon  at  which  they  begin  to  rise  would  differ 
much  in  different  species,  as  is  the  case.  Some  other 
agency,  however,  besides  apogeotropism,  must  come 
into  play,  either  directly  or  indirectly,  in  this  upward 
movement.  Thus  a  young  bean  (Viciafaba),  growing 
in  a  small  pot,  was  placed  in  front  of  a  window  in  a 
klinostat ;  and  at  night  the  leaves  rose  a  little,  although 


*  According  to  Frank  ('Die  organs  have  been  long  kept  in  the 
nat.  Wagerechte  Kichtung  von  dark,  the  amount  of  water  and  of 
Ptfanzentheilen,'  1870,  p.  46)  the  mineral  matter  which  they  con- 
root-leaves  of  many  plants,  kept  tain  is  so  much  altered,  and  their 
in  darkness,  rise  up  and  even  be-  regular  growth  is  so  much  dis- 
come  vertical ;  and  so  it  is  in  some  turbed,  that  it  is  perhaps  rash  to 
cases  with  shoots.  (See  Eauvven-  infer  from  their  movements  what 
hoff,  'Archives  Neerlandaises,'  would  occur  under  normal  con- 
torn,  xii.  p.  32.)  These  movements  ditions.  (See  Godlewski,  *Bot. 
indicate  apogeotropism  ;  but  when  Zeitung,'  Feb.  14th,  1879.) 


444  MODIFIED  CIRCUMNUTATION.          CHAP.  VIII. 

the  action  of  apogeotropism  was  quite  eliminated. 
Nevertheless,  they  did  not  rise  nearly  so  much  at 
night,  as  when  subjected  to  apogeotropism.  Is  it 
not  possible,  or  even  probable,  that  leaves  and  coty- 
ledons, which  have  moved  upwards  in  the  evening 
through  the  action  of  apogeotropism  during  countless 
generations,  may  inherit  a  tendency  to  this  movement  ? 
We  have  seen  that  the  hypocotyls  of  several  Legu- 
minous plants  have  from  a  remote  period  inherited  a 
tendency  to  arch  themselves ;  and  we  know  that  the 
sleep-movements  of  leaves  are  to  a  certain  extent 
inherited,  independently  of  the  alternations  of  light 
and  darkness. 

In  our  observations  on  the  circumnutation  of  those 
cotyledons  and  leaves  which  do  not  sleep  at  night,  we 
met  with  hardly  any  distinct  cases  of  their  sinking 
a  little  in  the  evening,  and  rising  again  in  the  morn- 
ing,— that  is,  of  movements  the  reverse  of  those  just 
discussed.  We  have  no  doubt  that  such  cases  occur, 
inasmuch  as  the  leaves  of  many  plants  sleep  by 
sinking  vertically  downwards.  How  to  account  for  the 
few  cases  which  were  observed  must  be  left  doubtful. 
The  young  leaves  of  Cannabis  sativa  sink  at  night 
between  30°  and  40°  beneath  the  horizon ;  and  Kraus 
attributes  this  to  epinasty  in  conjunction  with  the 
absorption  of  water.  Whenever  epinastic  growth  is 
vigorous,  it  might  conquer  diaheliotropism  in  the 
evening,  at  which  time  it  would  be  of  no  import- 
ance to  the  plant  to  keep  its  leaves  horizontal. 
The  cotyledons  of  Anoda  Wrightii,  of  one  variety  of 
Gossypium,  and  of  several  species  of  Ipomosa,  remain 
horizontal  in  the  evening  whilst  they  are  very  young ; 
as  they  grow  a  little  older  they  curve  a  little  down- 
wards, and  when  large  and  heavy  sink  so  much  that 
they  come  under  our  definition  of  sleep.  In  the  case  of 


CHAP.  VIII.  PARAHELIOTROPISM.  446 

the  Anoda  and  of  some  species  of  Ipomoea,  it  was  proved 
that  the  downward  movement  did  not  depend  on  the 
weight  of  the  cotyledons ;  but  from  the  fact  of  the  move- 
ment being  so  much  more  strongly  pronounced  after 
the  cotyledons  have  grown  large  and  heavy,  we  may 
suspect  that  their  weight  aboriginally  played  some  part 
in  determining  that  the  modification  of  the  circum- 
nutating  movement  should  be  in  a  downward  direction. 
The  so-called  Diurnal-  Sleep  of  Leaves,  or  ParaJielio- 
tropism. — This  is  another  class  of  movements,  dependent 
on  the  action  of  light,  which  supports  to  some  extent 
the  belief  that  the  movements  above  described  are 
only  indirectly  due  to  its  action.  We  refer  to  the 
movements  of  leaves  and  cotyledons  which  when 
moderately  illuminated  are  diahelio tropic ;  but  which 
change  their  positions  and  present  their  edges  to  the 
light,  when  the  sun  shines  brightly  on  them.  These 
movements  have  sometimes  been  called  diurnal  sleep, 
but  they  differ  wholly  with  respect  to  the  object 
gained  from  those  properly  called  nyctitropic  ;  and  in 
some  cases  the  position  occupied  during  the  day  is  the 
reverse  of  that  during  the  night. 

It  has  long  been  known*  that  when  the  sun  shines  brightly 
on  the  leaflets  of  Piobinia,  they  rise  up  and  present  their  edges 
to  the  light ;  whilst  their  position  at  night  is  vertically  down- 
wards. We  have  observed  the  same  movement,  when  the 
sun  shone  brightly  on  the  leaflets  of  an  Australian  Acacia. 
Those  of  AmphicarpcKa  monoica  turned  their  edges  to  the  sun ; 
and  an  analogous  movement  of  the  little  almost  rudimentary 
basal  leaflets  of  Mimosa  alUda  was  on  one  occasion  so  rapid  that 
it  could  be  distinctly  seen  through  a  lens.  The  elongated,  uni- 
foliate,  first  leaves  of  Phaseolus  Roxburgh™  stood  at  7  A.M.  at  20° 
above  the  horizon,  and  no  doubt  they  afterwards  sank  a  little 
lower.  At  noon,  after  having  been  exposed  for  about  2  h.  to 


*  Pfeffcr  gives  the  names  and  dates  of  several  ancient  writers  in  his 
;Die  Periodischen  Bewegungen,'  1875,  p,  62. 


446  MODIFIED   CIRCUMNUTATIOX.         CHAP.  VUL 

a  bright  sun,  they  stood  at  56°  above  the  horizon ;  they  were 
then  protected  from  the  rays  of  the  sun,  but  were  left  well 
illuminated  from  above,  and  after  30  m.  they  had  fallen  40°,  for 
they  now  stood  at  only  16°  above  the  horizon.  Some  young 
plants  of  Phaseolus  Eernandesii  had  been  exposed  to  the  same 
bright  sunlight,  and  their  broad,  tmifoliate,  first  leaves  now 
stood  up  almost  or  quite  vertically,  as  did  many  of  the  leaflets 
on  the  trifoliate  secondary  leaves ;  but  some  of  the  leaflets  had 
twisted  round  on  their  own  axes  by  as  much  as  90°  without 
rising,  so  as  to  present  their  edges  to  the  sun.  The  leaflets  on 
the  same  leaf  sometimes  behaved  in  these  two  different  manners, 
but  always  with  the  result  of  being  less  intensely  illuminated. 
These  plants  were  then  protected  from  the  sun,  and  were  looked 
at  after  1?  h. ;  and  now  all  the  leaves  and  leaflets  had  re- 
assumed  their  ordinary  sub-horizontal  positions.  The  copper- 
coloured  cotyledons  of  some  seedlings  of  Cassia  mimosoides  were 
horizontal  in  the  morning,  but  after  the  sun  had  shone  on 
them,  each  had  risen  45^°  above  the  horizon.  The  movement 
in  these  several  cases  must  not  be  confounded  with  the  sudden 
closing  of  the  leaflets  of  Mimosa  pudica,  which  may  sometimes 
be  noticed  when  a  plant  which  has  been  kept  in  an  obscure 
place  is  suddenly  exposed  to  the  sun  ;  for  in  this  case  the  light 
seems  to  act,  as  if  it  were  a  touch. 

From  Prof.  Wiesner's  interesting  observations,  it  is  probable 
that  the  above  movements  have  been  acquired  for  a  special 
purpose.  The  chlorophyll  in  leaves  is  often  -injured  by  too 
intense  a  light,  and  Prof.  Wiesner  *  believes  that  it  is  protected 
by  the  most  diversified  means,  such  as  the  presence  of  hairs, 
colouring  matter,  &c.,  and  amongst  other  means  by  the  leaves 
presenting  their  edges  to  the  sun,  so  that  the  blades  then 
receive  much  less  light.  He  experimented  on  the  young  leaflets 
of  Kobinia,  by  fixing  them  in  such  a  position  that  they  could 
not  escape  being  intensely  illuminated,  whilst  others  were 
allowed  to  place  themselves  obliquely;  and  the  former  began  to 
suffer  from  the  light  in  the  course  of  two  days. 

In  the  cases  above  given,  the  leaflets  move  either  upwards 


*  '  Die     Naturlichen    Einrich-  the  action  of  concentrated  light 

tungen  zum  Schutze  des  Chloro-  from  the  sun,  in  the  presence  of 

phylls,'   &c.,   1876.      Prin^sheim  oxygen.     See,  also,  Stahl  on  the 

has  recently  observed  under  the  protection    of    chlorophyll    from 

microscoj»e    the     destruction     of  intense  light,  in  'Bot.  Zeitung, 

chlorophyll  in  a  few  minutes  by  18SO. 


CHAP.  VIII.  PAKAHELIOTROPISM.  447 

or  twist  laterally,  so  as  to  place  their  edges  in  the  direction  of  the 
sun's  light;  but  Cohn  long  ago  observed  that  the  leaflets  of 
Oxalis  bend  downwards  when  fully  exposed  to  the  sun.  We 
witnessed  a  striking  instance  of  this  movement  in  the  very 
large  leaflets  of  0.  Orteyesii.  A  similar  movement  may  fre- 
quently be  observed  with  the  leaflets  of  Averrhoa  Ulimbi  (a 
member  of  the  Oxalidse) ;  and  a  leaf  is  here  represented  (Fig. 
180)  on  which  the  sun  had  shone.  A  diagram  (Fig.  134)  was 
given  in  the  last  chapter,  representing  the  oscillations  by  which 
a  leaflet  rapidly  descended  under  these  circumstances ;  and  the 
movement  may  be  seen  closely  to  resemble  that  (Fig.  133)  by 

Fig.  180. 


Averrhoa  bilimbi :  leaf  with  leaflets  depressed  after  exposure  to  sunshine ; 
but  the  leaflets  are  sometimes  more  depressed  than  is  here  shown. 
Figure  much  reduced. 

which  it  assumed  its  nocturnal  position.  It  is  an  interesting 
fact  in  relation  to  our  present  subject  that,  as  Prof.  Batalin 
informs  us  in  a  letter,  dated  February,  1879,  the  leaflets  of 
Oxalis  acctoselia  may  be  daily  exposed  to  the  sun  during  many 
weeks,  and  they  do  not  suffer  if  they  are  allowed  to  depress 
themselves ;  but  if  this  be  prevented,  they  lose  their  colour  and 
wither  in  two  or  three  days.  Yet  the  duration  of  a  leaf  is  about 
two  months,  when  subjected  only  to  diffused  light ;  and  in  this 
case  the  leaflets  never  sink  downwards  during  the  day 

As  the  upward  movements  of  the  leaflets  of  Kobinia, 
and  the  downward  movements  of  those  of  Oxalis,  have 
been  proved  to  be  highly  beneficial  to  these  plants 
when  subjected  to  bright  sunshine,  it  seems  probable 
that  they  have  been  acquired  for  the  special  purpose 
of  avoiding  too  intense  an  illumination.  As  it  would 
have  been  very  troublesome  in  all  the  above  cases  to 
20 


448  MODIFIED  CIRCUMNTJTATIOX.         CHAP.  VIH. 

have  watched  for  a  fitting  opportunity  and  to  have 
traced  the  movement  of  the  leaves  whilst  they  were 
fully  exposed  to  the  sunshine,  we  did  not  ascertain 
whether  paraheliotropism  always  consisted  of  modi- 
fied circumnutation ;  but  this  certainly  was  the  case 
with  the  Averrhoa,  and  probably  with  the  other  species, 
as  their  leaves  were  continually  circumnutating. 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT,  449 


CHAPTER  IX. 

SENSITIVENESS  OF  PLANTS  TO  LIGHT:  ITS  TRANSMITTED  EFFECTS. 

Uses  of  heliotropism — Insectivorous  and  climbing  plants  not  heliotropic 
— Same  organ  heliotropic  at  one  age  and  not  at  another — Extra- 
ordinary sensitiveness  of  some  plants  to  light — The  effects  of  light  do 
not  correspond  with  its  intensity—  Effects  of  previous  illumination 
— Time  required  for  the  action  of  light— After-effects  of  light — 
Apogeotropi?m  acts  as  soon  as  light  fails — Accuracy  with  which 
plants  bend  to  the  light — This  dependent  on  the  illumination  of 
one  whole  side  of  the  part — Localised  sensitiveness  to  light  and  its 
transmitted  effects — Cotyledons  of  1  halaris,  manner  of  bending — 
Results  of  the  exclusion  of  light  from  their  tips — Effects  trans- 
mitted beneath  the  surface  of  the  ground — Lateral  illumination  of 
the  tip  determines  the  direction  of  the  curvature  of  the  base— Coty- 
ledons of  Avena,  curvature  of  basal  part  due  to  the  illumination  of 
upper  part — Similar  results  with  the  hypocotyls  of  Brassica  and 
Beta — Radicles  of  Sinapis  apheliotropic,  due  to  the  sensitiveness  of 
their  tips — Concluding  remarks  and  summary  of  chapter — Means 
by  which  circumnutation  has  been  converted  into  heliotropism  or 
apheliotropisin. 

No  one  can  look  at  the  plants  growing  on  a  bank  or 
on  the  borders  of  a  thick  wood,  and  doubt  that  the 
young  stems  and  leaves  place  themselves  so  that  the 
leaves  may  be  well  illuminated.  They  are  thus  enabled 
to  decompose  carbonic  acid.  But  the  sheath-like  coty- 
ledons of  some  Gramineae,  for  instance,  those  of  Pha- 
laris,  are  not  green  and  contain  very  little  starch ; 
from  which  fact  we  may  infer  that  they  decompose 
little  or  no  carbonic  acid.  Nevertheless,  they  are  ex- 
tremely heliotropic ;  and  this  probably  serves  them  in 
another  way,  namely,  as  a  guide  from  the  buried  seeds 
through  fissures  in  the  ground  or  through  overlying 
masses  of  vegetation,  into  the  light  and  air.  This  view 


450  SENSITIVENESS   TO  LIGHT.  CHAP.  IX. 

is  strengthened  by  the  fact  that  with  Phalaris  and 
Avena  the  first  true  leaf,  which  is  bright  green  and  no 
doubt  decomposes  carbonic  acid,  exhibits  hardly  a 
trace  of  heliotropism.  The  heliotropic  movements  of 
many  other  seedlings  probably  aid  them  in  like 
manner  in  emerging  from  the  ground  ;  for  apogeo- 
tropism  by  itself  would  blindly  guide  them  upwards, 
against  any  overlying  obstacle. 

Heliotropism  prevails  so  extensively  among  the 
higher  plants,  that  there  are  extremely  few,  of  which 
some  part,  either  the  stem,  flower-peduncle,  petiole, 
or  leaf,  does  not  bend  towards  a  lateral  light. 
Drosera  rotundifolia  is  one  of  the  few  plants  the 
leaves  of  which  exhibit  no  trace  of  heliotropism.  Nor 
could  we  see  any  in  Dionaea,  though  the  plants  were 
not  so  carefully  observed.  Sir  J.  Hooker  exposed  the 
pitchers  of  Sarracenia  for  some  time  to  a  lateral  light, 
but  they  did  not  bend  towards  it.*  We  can  understand 
the  reason  why  these  insectivorous  plants  should  not 
be  heliotropic,  as  they  do  not  live  chiefly  by  decom- 
posing carbonic  acid  ;  and  it  is  much  more  important 
to  them  that  their  leaves  should  occupy  the  best 
position  for  capturing  insects,  than  that  they  should 
be  fully  exposed  to  the  light. 

Tendrils,  which  consist  of  leaves  or  of  other  organs 
modified,  and  the  stems  of  twining  plants,  are,  as 
Mohl  long  ago  remarked,  rarely  heliotropic ;  and  here 
again  we  can  see  the  reason  why,  for  if  they  had 
moved  towards  a  lateral  light  they  would  have  been 
drawn  away  from  their  supports.  But  some  tendrils  are 
apheliotropic,  for  instance  those  of  Bignonia  capreolata 


*  According  to  F.  Kurlz  ('  Ver-  tonia    Californira    are    strongly 

handl.  des  Bot.  Vereins  der  Pro-  apheliotropic.   We  failed  to  detect 

vinz  Brandenburg/  Bd.  xx.  1878)  this  movement  in  a  plant  which 

the  leaves  or  pitchers  of  Darling-  we  possessed  for  a  short  time. 


CHAP.  IX.  SENSITIVENESS  TO   LIGHT.  451 

and  of  Smilax  aspera  ;  and  the  stems  of  some  plants 
which  climb  by  rootlets,  as  those  of  the  Ivy  and  Tecoma 
radicans,  are  likewise  apheliotropic,  and  they  thus  find 
a  support.  The  leaves,  on  the  other  hand,  of  most 
climbing  plants  are  heliotropic  ;  but  we  could  detect 
no  signs  of  any  such  movement  in  those  of  Mutisia 
clematis. 

As  heliotropism  is  so  widely  prevalent,  and  as 
twining  plants  are  distributed  throughout  the  whole 
vascular  series,  the  apparent  absence  of  any  tendency 
in  their  stems  to  bend  towards  the  light,  seemed  to 
us  so  remarkable  a  fact  as  to  deserve  further  in- 
vestigation, for  it  implies  that  heliotropism  can  be 
readily  eliminated.  When  twining  plants  are  exposed 
to  a  lateral  light,  their  stems  go  on  revolving  or  cir- 
cumnutating  about  the  same  spot,  without  any  evident 
deflection  towards  the  light ;  but  we  thought  that 
we  might  detect  some  trace  of  heliotropism  by  com- 
paring the  average  rate  at  which  the  stems  moved  to 
and  from  the  light  during  their  successive  revolutions.* 
Three  young  plants  (about  a  foot  in  height)  of  Ipomoea 
cterulea  and  four  of  J.  purpurea,  growing  in  separate 
pots,  were  placed  on  a  bright  day  before  a  north-east 
window  in  a  room  otherwise  darkened,  with  the  tips 
of  their  revolving  stems  fronting  the  window.  When 
the  tip  of  each  plant  pointed  directly  from  the  window, 
and  when  again  towards  it,  the  times  were  recorded. 
This  was  continued  from  6.45  A.M.  till  a  little  after 
2  P.M.  on  June  17th.  After  a  few  observations  we 
concluded  that  we  could  safely  estimate  the  time 


*  Some  erroneous  statements  number  of  observations,  for  we  did 
are  unfortunately  given  on  this  not  then  know  at  how  unequal 
subject,  in  *  The  Movements  and  a  rate  the  stems  and  tendrils  of 
Habits  of  Climbing  Plants,'  1875,  climbing  plants  sometimes  travel 
pp.  28,32,  40,  and  53.  Conclusions  in  different  parts  of  the  same  re- 
were  drawn  from  an  insufficient  volution. 


452  SENSITIVENESS  TO  LIGHT,  CHAP.  IX. 

taken  by  each  semicircle,  within  a  limit  of  error  of  at 
most  5  minutes.  Although  the  rate  of  movement  in 
different  parts  of  the  same  revolution  varied  greatly, 
yet  22  semicircles  to  the  light  were  completed,  each 
on  an  average  in  73'95  minutes;  and  22  semicircles 
from  the  light  each  in  73*5  minutes.  It  may,  there- 
fore, be  said  that  they  travelled  to  and  from  the  light 
at  exactly  the  same  average  rate  ;  though  probably 
the  accuracy  of  the  result  was  in  part  accidental.  In 
the  evening  the  stems  were  not  in  the  least  deflected 
towards  the  window.  Nevertheless,  there  appears  to 
exist  a  vestige  of  heliotropism,  for  with  6  out  of  the 
7  plants,  the  first  semicircle  from  the  light,  described 
in  the  early  morning  after  they  had  been  subjected  to 
darkness  during  the  night  and  thus  probably  rendered 
more  sensitive,  required  rather  more  time,  and  the  first 
semicircle  to  the  light  considerably  less  time,  than  the 
average.  Thus  with  all  7  plants,  taken  together,  the 
mean  time  of  the  first  semicircle  in  the  morning  from 
the  light,  was  76*8  minutes,  instead  of  73'5  minutes, 
which  is  the  mean  of  all  the  semicircles  during  the 
day  from  the  light ;  and  the  mean  of  the  first  semi- 
circle to  the  light  was  only  63'1,  instead  of  73*95 
minutes,  which  was  the  mean  of  all  the  semicircles 
during  the  day  to  the  light. 

Similar  observations  were  made  on  Wistaria  Sinensis, 
and  the  mean  of  9  semicircles  from  the  light  was 
117  minutes,  and  of  7  semicircles  to  the  light  122 
minutes,  and  this  difference  does  not  exceed  the  pro- 
bable limit  of  error.  During  the  three  days  of  expo- 
sure, the  shoot  did  not  become  at  all  bent  towards  the 
window  before  which  it  stood.  In  this  case  the  first 
semicircle  from  the  light  in  the  early  morning  of  each 
day,  required  rather  less  time  for  its  performance  than 
did  the  first  semicircle  to  the  light ;  and  this  result, 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT.  453 

if  not  accidental,  appears  to  indicate  that  the  shoots 
retain  a  trace  of  an  original  apheliotropic  tendency. 
With  Lonicera  Irachypoda  the  semicircles  from  and  to 
the  light  differed  considerably  in  time ;  for  5  semi- 
circles from  the  light  required  on  a  mean  202'4 
minutes,  and  4  to  the  light,  229*5  minutes ;  but  the 
shoot  moved  very  irregularly,  and  under  these  circum- 
stances the  observations  were  much  too  few. 

It  is  remarkable  that  the  same  part  on  the  same 
plant  may  be  affected  by  light  in  a  widely  different 
manner  at  different  ages,  and  as  it  appears  at  different 
seasons.  The  hypocotyledonous  stems  of  Ipomcea 
cserulca  and  purpurea  are  extremely  heliotropic,  whilst 
the  stems  of  older  plants,  only  about  a  foot  in  height, 
are,  as  we  have  just  seen,  almost  wholly  insensible  to 
light.  Sachs  states  (and  we  have  observed  the  same 
fact)  that  the  hypocotyls  of  the  Ivy  (Hedera  helix)  are 
slightly  heliotropic ;  whereas  the  stems  of  plants  grown 
to  a  few  inches  in  height  become  so  strongly  aphelio- 
tropic, that  they  bend  at  right  angles  away  from  the 
light.  Nevertheless,  some  young  plants  which  had 
behaved  in  this  manner  early  in  the  summer  again 
became  distinctly  heliotropic  in  the  beginning  of 
September ;  and  the  zigzag  courses  of  their  stems,  as 
they  slowly  curved  towards  a  north-east  window,  were 
traced  during  10  days.  The  stems  of  very  young 
plants  of  Tropneolum  majus  are  highly  heliotropic,  whilst 
those  of  older  plants,  according  to  Sachs,  are  slightly 
apheliotropic.  In  all  these  cases  the  heliotropism  of 
the  very  young  stems  serves  to  expose  the  cotyledons, 
or  when  the  cotyledons  are  hypogean  the  first  true 
leaves,  fully  to  the  light ;  and  the  loss  of  this  power 
by  the  older  stems,  or  their  becoming  apheliotropic, 
is  connected  with  their  habit  of  climbing. 

Most  seedling  plants  are  strongly  heliotropic,  and 


454  SENSITIVENESS  TO   LIGHT.  CHAP.  IX 

it  is  no  doubt  a  great  advantage  to  them  in  their 
struggle  for  life  to  expose  their  cotyledons  to  the 
light  as  quickly  and  as  fully  as  possible,  for  the  sake 
of  obtaining  carbon.  It  has  been  shown  in  the  first 
chapter  that  the  greater  number  of  seedlings  circum- 
nutate  largely  and  rapidly ;  and  as  heliotropism  con- 
sists of  modified  circumnutation,  we  are  tempted  to 
look  at  the  high  development  of  these  two  powers  in 
seedlings  as  intimately  connected.  Whether  there  are 
any  plants  which  circumnutate  slowly  and  to  a  small 
extent,  and  yet  are  highly  heliotropic,  we  do  not 
know ;  but  there  are  several,  and  there  is  nothing 
surprising  in  this  fact,  which  circumnutate  largely  and 
are  not  at  all,  or  only  slightly,  heliotropic.  Of  such 
cases  Drosera  rotundifolia  offers  an  excellent  instance. 
The  stolons  of  the  strawberry  circumnutate  almost 
like  the  stems  of  climbing  plants,  and  they  are  not  at 
all  affected  by  a  moderate  light;  but  when  exposed 
late  in  the  summer  to  a  somewhat  brighter  light  they 
were  slightly  heliotropic;  in  sunlight,  according  to 
De  Vries,  they  are  apheliotropic.  Climbing  plants 
circumnutate  much  more  widely  than  any  other  plants, 
yet  they  are  not  at  all  heliotropic. 

Although  the  stems  of  most  seedling  plants  are 
strongly  heliotropic,  some  few  are  but  slightly  helio- 
tropic, without  our  being  able  to  assign  any  reason. 
This  is  the  case  with  the  hypocotyl  of  Cassia  tora,  and 
we  were  struck  with  the  same  fact  with  some  other 
seedlings,  for  instance,  those  of  Reseda  odorata.  With 
respect  to  the  degree  of  sensitiveness  of  the  more 
sensitive  kinds,  it  was  shown  in  the  last  chapter  that 
seedlings  of  several  species,  placed  before  a  north-east 
window  protected  by  several  blinds,  and  exposed  in 
the  rear  to  the  diffused  light  of  the  room,  moved 
with  unerring  certainty  towards  the  window,  although 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT.  455 

it  was  impossible  to  judge,  excepting  by  the  shadow 
cast  by  an  upright  pencil  on  a  white  card,  on  which 
side  most  light  entered,  so  that  the  excess  on  one  side 
must  have  been  extremely  small. 

A  pot  with  seedlings  of  Phalaris  Canariensis,  which 
had  been  raised  in  darkness,  was  placed  in  a  com- 
pletely darkened  room,  at  12  feet  from  a  very  small 
lamp.  After  3  h.  the  cotyledons  were  doubtfully 
curved  towards  the  light,  and  after  7  h.  40  m.  from 
the  first  exposure,  they  were  all  plainly,  though 
slightly,  curved  towards  the  lamp.  Now,  at  this  dis- 
tance of  12  feet,  the  light  was  so  obscure  that  we  could 
not  see  the  seedlings  themselves,  nor  read  the  large 
Eoman  figures  on  the  white  face  of  a  watch,  nor  see  a 
pencil  line  on  paper,  but  could  just  distinguish  a  line 
made  with  Indian  ink.  It  is  a  more  surprising  fact 
that  no  visible  shadow  was  cast  by  a  pencil  held 
upright  on  a  white  card;  the  seedlings,  therefore, 
were  acted  on  by  a  difference  in  the  illumination  of 
their  two  sides,  which  the  human  eye  could  not  dis- 
tinguish. On  another  occasion  even  a  less  degree  of 
light  acted,  for  some  cotyledons  of  Phalaris  became 
slightly  curved  towards  the  same  lamp  at  a  distance 
of  20  feet;  at  this  distance  we  could  not  see  a  cir- 
cular dot  2-29  mm.  (*09  inch)  in  diameter  made  with 
Indian  ink  on  white  paper,  though  we  could  just  see  a 
dot  3*56  mm.  (-14  inch)  in  diameter;  yet  a  dot  of 
the  former  size  appears  large  when  seen  in  the  light.* 

We  next  tried  how  small  a  beam  of  light  would  act ; 
as  this  bears  on  light  serving  as  a  guide  to  seedlings 
whilst  they  emerge  through  fissured  or  encumbered 
ground.  A  pot  with  seedlings  of  Phalaris  was  covered 


*  Strasburgor  says  ('  Wirkung  Hsematococcus  moved  to  a  light 
des  Lichtes  auf  Schwarmsporen,'  which  only  just  sufficed  to  allow 
1878,  p.  52),  that  the  spores  of  middle-sized  type  to  be  read. 


4:56  SENSITIVENESS  TO  LIGHT.  CHAP.  IX. 

by  a  tin-vessel,  having  on  one  side  a  circular  hole 
1*23  mm.  in  diameter  (i.e.  a  little  less  than  the  -jVtk  of 
an  inch)  ;  and  the  box  was  placed  in  front  of  a  paraffin 
lamp  and  on  another  occasion  in  front  of  a  window  ; 
and  both  times  the  seedlings  were  manifestly  bent 
after  a  few  hours  towards  the  little  hole. 

A  more  severe  trial  was  now  made ;  little  tubes  of 
very  thin  glass,  closed  at  their  upper  ends  and  coated 
with  black  varnish,  were  slipped  over  the  cotyledons 
of  Phalaris  (which  had  germinated  in  darkness)  and 
just  fitted  them.  Narrow  stripes  of  the  varnish  had 
been  previously  scraped  off  one  side,  through  which 
alone  light  could  enter ;  and  their  dimensions  were 
afterwards  measured  under  the  microscope.  As  a 
control  experiment,  similar  unvarnished  and  trans- 
parent tubes  were  tried,  and  they  did  not  prevent  the 
cotyledons  bending  towards  the  light.  Two  cotyledons 
were  placed  before  a  south-west  window,  one  of  which 
was  illuminated  by  a  stripe  in  the  varnish,  only  '004 
inch  (Ol  mm.)  in  breadth  and  "016  inch  (0*4  mm.)  in 
length  ;  and  the  other  by  a  stripe  '008  inch  in  breadth 
and  '06  inch  in  length.  The  seedlings  were  examined 
after  an  exposure  of  7  h.  40  m.,  and  were  found  to  be 
manifestly  bowed  towards  the  light.  Some  other  coty- 
ledons were  at  the  same  time  treated  similarly,  ex- 
cepting that  the  little  stripes  were  directed  not  to  the 
sky,  but  in  such  a  manner  that  they  received  only  the 
diffused  light  from  the  room  ;  and  these  cotyledons  did 
not  become  at  all  bowed.  Seven  other  cotyledons  were 
illuminated  through  narrow,  but  comparatively  long, 
cleared  stripes  in  the  varnish — namely,  in  breadth 
between  '01  and  '026  inch,  and  in  length  between  '15 
and  *3  inch  ;  and  these  all  became  bowed  to  the  side, 
by  which  light  entered  through  the  stripes,  whether 
these  were  directed  towards  the  sky  or  to  one  side  of 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT.  457 

the  room.  That  light  passing  through  a  hole  only 
•004  inch  in  breadth  by  '016  in  length,  should  induce 
curvature,  seems  to  us  a  surprising  fact. 

Before  we  knew  how  extremely  sensitive  the  coty- 
ledons of  Phalaris  were  to  light,  we  endeavoured  to 
trace  their  circumnutation  in  darkness  by  the  aid  of 
a  small  wax  taper,  held  for  a  minute  or  two  at  each 
observation  in  nearly  the  same  position,  a  little  on  the 
left  side  in  front  of  the  vertical  glass  on  which  the 
tracing  was  made.  The  seedlings  were  thus  observed 
seventeen  times  in  the  course  of  the  day,  at  intervals  of 
from  half  to  three-quarters  of  an  hour ;  and  late  in  the 
evening  we  were  surprised  to  find  that  all  the  29  coty- 
ledons were  greatly  curved  and  pointed  towards  the 
vertical  glass,  a  little  to  the  left  where  the  taper  had 
been  held.  The  tracings  showed  that  they  had  tra- 
velled in  zigzag  lines.  Thus,  an  exposure  to  a  feeble 
light  for  a  very  short  time  at  the  above  specified 
intervals,  sufficed  to  induce  well-marked  heliotropism. 
An  analogous  case  was  observed  with  the  hypocotyls 
of  Soldnum  lycopersicum.  We  at  first  attributed  this 
result  to  the  after-effects  of  the  light  on  each  occasion  ; 
but  since  reading  Wiesner's  observations,*  which  will 
be  referred  to  in  the  last  chapter,  we  cannot  doubt  that 
an  intermittent  light  is  more  efficacious  than  a  con- 
tinuous one,  as  plants  are  especially  sensitive  to  any 
contrast  in  its  amount. 

The  cotyledons  of  Phalaris  bend  much  more  slowly 
towards  a  very  obscure  light  than  towards  a  bright 
one.  Thus,  in  the  experiments  with  seedlings  placed 
in  a  dark  room  at  12  feet  from  a  very  small  lamp,  they 
were  just  perceptibly  and  doubtfully  curved  towards  it 
after  3  h.,  and  only  slightly,  yet  certainly,  after  4  h. 

*  '  Sitz.  der  k.  Akad.  dcr  WissenscL.'  (Vienna),  Jan.  1880,  p.  12. 


458  SENSITIVENESS  TO   LIGHT.  CHAP.  IX. 

After  8  h.  40  m.  the  chords  of  their  arcs  were  deflected 
from  the  perpendicular  by  an  average  angle  of  only 
16°.  Had  the  light  been  bright,  they  would  have 
become  much  more  curved  in  between  1  and  2  h. 
Several  trials  were  made  with  seedlings  placed  at 
various  distances  from  a  small  lamp  in  a  dark  room ; 
but  we  will  give  only  one  trial.  Six  pots  were  placed 
at  distances  of  2,  4,  8,  12,  16,  and  20  feet  from  the 
lamp,  before  which  they  were  left  for  4  h.  As  light 
decreases  in  a  geometrical  ratio,  the  seedlings  in  the 
2nd  pot  received  -\ih,  those  in  the  3rd  pot  -^ik, 
those  in  the  4th  -jVth,  those  in  the  5th  FVth,  and  those 
in  the  6th  -r^-th  of  the  light  received  by  the  seedlings  in 
the  first  or  nearest  pot.  Therefore  it  might  have  been 
expected  that  there  would  have  been  an  immense  differ- 
ence in  the  degree  of  their  heliotropic  curvature  in  the 
several  pots ;  and  there  was  a  well-marked  difference 
between  those  which  stood  nearest  and  furthest  from 
the  lamp,  but  the  difference  in  each  successive  pair  of 
pots  was  extremely  small.  In  order  to  avoid  prejudice, 
we  asked  three  persons,  who  knew  nothing  about  the 
experiment,  to  arrange  the  pots  in  order  according  to 
the  degree  of  curvature  of  the  cotyledons.  The  first 
person  arranged  them  in  proper  order,  but  doubted 
long  between  the  12  feet  and  16  feet  pots  ;  yet  these 
two  received  light  in  the  proportion  of  36  to  64.  The 
second  person  also  arranged  them  properly,  but 
doubted  between  the  8  feet  and  12  feet  pots,  which 
received  light  in  the  proportion  of  16  to  36.  The 
third  person  arranged  them  in  wrong  order,  and 
doubted  about  four  of  the  pots.  This  evidence  shows 
conclusively  how  little  the  curvature  of  the  seedlings 
differed  in  the  successive  pots,  in  comparison  with  the 
great  difference  in  the  amount  of  light  which  they 
received;  and  it  should  be  noted  that  there  was  no 


CHAP.  IX.  SENSITIVENESS   TO  LIGHT.  459 

excess  of  superfluous  light,  for  the  cotyledons  became 
but  little  and  slowly  curved  even  in  the  nearest  pot. 
Close  to  the  6th  pot,  at  the  distance  of  20  feet  from 
the  lamp,  the  light  allowed  us  just  to  distinguish 
a  dot  3'56  mm.  (-14  inch)  in  diameter,  made  with 
Indian  ink  on  white  paper,  but  not  a  dot  2*29  mm. 
(•09  inch)  in  diameter. 

The  degree  of  curvature  of  the  cotyledons  of  Phalaris 
within  a  given  time,  depends  not  merely  on  the 
amount  of  lateral  light  which  they  may  then  receive, 
but  on  that  which  they  have  previously  received  from 
above  and  on  all  sides.  Analogous  facts  have  been 
given  with  respect  to  the  nyctitropic  and  periodic 
movements  of  plants.  Of  two  pots  containing  seedlings 
of  Phalaris  which  had  germinated  in  darkness,  one  was 
still  kept  in  the  dark,  and  the  other  was  exposed  (Sept. 
26th)  to  the  light  in  a  greenhouse  during  a  cloudy  day 
and  on  the  following  bright  morning.  On  this  morn- 
ing (27th),  at  10.30  A.M.,  both  pots  were  placed  in  a 
box,  blackened  within  and  open  in  front,  before  a 
north-east  window,  protected  by  a  linen  and  muslin 
blind  and  by  a  towel,  so  that  but  little  light  was 
admitted,  though  the  sky  was  bright.  Whenever  the 
pots  were  looked  at,  this  was  done  as  quickly  as  pos- 
sible, and  the  cotyledons  were  then  held  transversely 
with  respect  to  the  light,  so  that  their  curvature  could 
not  have  been  thus  increased  or  diminished.  After 
50  m.  the  seedlings  which  had  previously  been  kept 
in  darkness,  were  perhaps,  and  after  70  m.  were  cer- 
tainly, curved,  though  very  slightly,  towards  the 
window.  After  85  m.  some  of  the  seedlings,  which 
had  previously  been  illuminated,  were  perhaps  a  little 
affected,  and  after  100  m.  some  of  the  younger  ones 
were  certainly  a  little  curved  towards  the  light.  At 
this  time  (i.e.  after  100  m.)  there  was  a  plain  difference 


460  SENSITIVENESS   TO   LIGHT.  CHAF.  IX. 

in  the  curvature  of  the  seedlings  in  the  two  pots. 
After  2  h.  12  m.  the  chords  of  the  arcs  of  four  of 
the  most  strongly  curved  seedlings  in  each  pot  were 
measured,  and  the  mean  angle  from  the  perpendicular 
of  those  which  had  previously  been  kept  in  darkness 
was  19°,  and  of  those  which  had  previously  been  illu- 
minated only  7°.  Nor  did  this  difference  diminish 
during  two  additional  hours.  As  a  check,  the  seed- 
lings in  both  pots  were  then  placed  in  complete  dark- 
ness for  two  hours,  in  order  that  apogeotropism  should 
act  on  them;  and  those  in  the  one  pot  which  were 
little  curved  became  in  this  time  almost  completely 
upright,  whilst  the  more  curved  ones  in  the  other  pot 
still  remained  plainly  curved. 

Two  days  afterwards  the  experiment  was  repeated, 
with  the  sole  difference  that  even  less  light  was 
admitted  through  the  window,  as  it  was  protected  by  a 
linen  and  muslin  blind  and  by  two  towels  ;  the  sky, 
moreover,  was  somewhat  less  bright.  The  result  was 
the  same  as  before,  excepting  that  everything  occurred 
rather  slower.  The  seedlings  which  had  been  pre- 
viously kept  in  darkness  were  not  in  the  least  curved 
after  54  m.,  but  were  so  after  70  m.  Those  which  had 
previously  been  illuminated  were  not  at  all  affected 
until  130  in.  had  elapsed,  and  then  only  slightly. 
After  145  m.  some  of  the  seedlings  in  this  latter  pot 
were  certainly  curved  towards  the  light ;  and  there 
was  now  a  plain  difference  between  the  two  pots.  After 
3  h.  45  m.  the  chords  of  the  arcs  of  3  seedlings  in 
each  pot  were  measured,  and  the  mean  angle  from  the 
perpendicular  was  16°  for  those  in  the  pot  which  had 
previously  been  kept  in  darkness,  and  only  5°  for 
those  which  had  previously  been  illuminated. 

The  curvature  of  the  cotyledons  of  Phalaris  towards 
a  lateral  light  is  therefore  certainly  influenced  by  the 


CHAP.  IX.  SENSITIVENESS   TO   LIGHT.  461 

degree  to  which  they  have  been  previously  illu- 
minated. We  shall  presently  see  that  the  influence 
of  light  on  their  bending  continues  for  a  short  time 
after  the  light  has  been  extinguished.  These  facts,  as 
well  as  that  of  the  curvature  not  increasing  or  de- 
creasing in  nearly  the  same  ratio  with  that  of  the 
amount  of  light  which  they  receive,  as  shown  in  the 
trials  with  the  plants  before  the  lamp,  all  indicate 
that  light  acts  on  them  as  a  stimulus,  in  somewhat 
the  same  manner  as  on  the  nervous  system  of  animals, 
and  not  in  a  direct  manner  on  the  cells  or  cell- walls 
which  by  their  contraction  or  expansion  cause  the 
curvature. 

It  has  already  been  incidentally  shown  how  slowly 
the  cotyledons  of  Phalaris  bend  towards  a  very  dim 
light ;  but  when  they  were  placed  before  a  bright 
paraffin  lamp  their  tips  were  all  curved  rectangularly 
towards  it  in  2  h.  20  m.  The  hypocotyls  of  Solanum 
lycopersicum  had  bent  in  the  morning  at  right  angles 
to  wards  a  north -east  window.  At  1  P.M.  (Oct.  21st)  the 
pot  was  turned  round,  so  that  the  seedlings  now  pointed 
from  the  light,  but  by  .5  P.M.  they  had  reversed  their 
curvature  and  again  pointed  to  the  light.  They  had 
thus  passed  through  180°  in  4  h.,  having  in  the 
morning  previously  passed  through  about  90°.  But  the 
reversal  of  the  first  half  of  the  curvature  will  have 
been  aided  by  apogeotropism.  Similar  cases  were 
observed  with  other  seedlings,  for  instance,  with  those 
of  Sinapis  alba. 

We  attempted  to  ascertain  in  how  short  a  time 
light  acted  on  the  cotyledons  of  Phalaris,  but  this 
was  difficult  on  account  of  their  rapid  circumnutating 
movement ;  moreover,  they  differ  much  in  sensibility, 
according  to  age ;  nevertheless,  some  of  our  observa- 
tions are  worth  giving.  Pots  with  seedlings  were 


462  SENSITIVENESS   TO  LIGHT.  CHAP.  IX. 

placed  under  a  microscope  provided  with  an  eye-piece 
micrometer,  of  which  each  division  equalled  -^-o-th  of  an 
inch  (0'051  mm.)  ;  and  they  were  at  first  illuminated 
by  light  from  a  paraffin  lamp  passing  through  a  solu- 
tion of  bichromate  of  potassium,  which  does  not  induce 
heliotropism.  Thus  the  direction  in  which  the  coty- 
ledons were  circumnutating  could  be  observed  inde- 
pendently of  any  action  from  the  light ;  and  they  could 
be  made,  by  turning  round  the  pots,  to  circumnutate 
transversely  to  the  line  in  which  the  light  would  strike 
them,  as  soon  as  the  solution  was  removed.  The  fact 
that  the  direction  of  the  circumnutating  movement 
might  change  at  any  moment,  and  thus  the  plant 
might  bend  either  towards  or  from  the  lamp  indepen- 
dently of  the  action  of  the  light,  gave  an  element  of 
uncertainty  to  the  results.  After  the  solution  had 
been  removed,  five  seedlings  which  were  circumnutat- 
ing transversely  to  the  line  of  light,  began  to  move 
towards  it,  in  6,  4,  7£,  6,  and  9  minutes.  In  one  of 
these  cases,  the  apex  of  the  cotyledon  crossed  five 
of  the  divisions  of  the  micrometer  (i.e.  j^oth  of  an 
inch,  or  0*254  mm.)  towards  the  light  in  3  m.  Of  two 
seedlings  which  were  moving  directly  from  the  light  at 
the  time  when  the  solution  was  removed,  one  began  to 
move  towards  it  in  13  m.,  and  the  other  in  15  m. 
This  latter  seedling  was  observed  for  more  than  an 
hour  and  continued  to  move  towards  the  light;  it 
crossed  at  one  time  5  divisions  of  the  micrometer 
(0*254  mm.)  in  2  m.  30  s.  In  all  these  cases,  the 
movement  towards  the  light  was  extremely  unequal  in 
rate,  and  the  cotyledons  often  remained  almost  sta- 
tionary for  some  minutes,  and  two  of  them  retrograded 
a  little.  Another  seedling  which  was  circumnutating 
transversely  to  the  line  of  light,  moved  towards,  it  in 
4  m.  after  the  solution  was  removed ;  it  then  remained 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT.  463 

almost  stationary  for  10  m. ;  then  crossed  5  divisions 
of  the  micrometer  in  6  m. ;  and  then  8  divisions  in 
11  m.  This  unequal  rate  of  movement,  interrupted 
by  pauses,  and  at  first  with  occasional  retrogressions, 
accords  well  with  our  conclusion  that  heliotropism 
consists  of  modified  circumnutation. 

In  order  to  observe  how  long  the  after-effects  of 
light  lasted,  a  pot  with  seedlings  of  Phalaris,  which 
had  germinated  in  darkness,  was  placed  at  10.40  A.M. 
before  a  north-east  window,  being  protected  on  all 
other  sides  from  the  light;  and  the  movement  of  a 
cotyledon  was  traced  on  a  horizontal  glass.  It  cir- 
cumnutated  about  the  same  space  for  the  first  24  m., 
and  during  the  next  1  h.  33  m.  moved  rapidly  towards 
the  light.  The  light  was  now  (i.e.  after  1  h.  57  m.) 
completely  excluded,  but  the  cotyledon  continued 
bending  in  the  same  direction  as  before,  certainly  for 
more  than  15  m.,  probably  for  about  27  m.  The  doubt 
arose  from  the  necessity  of  not  looking  at  the  seed- 
lings often,  and  thus  exposing  them,  though  momen- 
tarily, to  the  light.  This  same  seedling  was  now  kept 
in  the  dark,  until  2.18  P.M.,  by  which  time  it  had 
reacquired  through  apogeotropism  its  original  upright 
position,  when  it  was  again  exposed  to  the  light  from 
a  clouded  sky.  By  3  P.M.  it  had  moved  a  very  short 
distance  towards  the  light,  but  during  the  next  45  m. 
travelled  quickly  towards  it.  After  this  exposure  of 
1  h.  27  m.  to  a  rather  dull  sky,  the  light  was  again 
completely  excluded,  but  the  cotyledon  continued  to 
bend  in  the  same  direction  as  before  for  14  m.  within 
a  very  small  limit  of  error.  It  was  then  placed  in 
the  dark,  and  it  now  moved  backwards,  so  that  after 
1  h.  7  m.  it  stood  close  to  where  it  had  started  from  at 
2.18  P.M.  These  observations  show  that  the  coty- 
ledons of  Phalaris,  after  being  exposed  to  a  lateral 


464  SENSITIVENESS   TO  LIGHT.  CHAP.  XX. 

light,   continue   to   bend   in  the   same  direction  for 
between  a  quarter  and  half  an.  hour. 

In  the  two  experiments  just  given,  the  cotyledons 
moved  backwards  or  from  the  window  shortly  after 
being  subjected  to  darkness;  and  whilst  tracing  the 
circumnutation  of  various  kinds  of  seedlings  exposed 
to  a  lateral  light,  we  repeatedly  observed  that  late  in 
the  evening,  as  the  light  waned,  they  moved  from  it. 
This  fact  is  shown  in  some  of  the  diagrams  given  in 
the  last  chapter.  We  wished  therefore  to  learn  whether 
this  was  wholly  due  to  apogeotropism,  or  whether  an 
organ  after  bending  towards  the  light  tended  from 
any  other  cause  to  bend  from  it,  as  soon  as  the  light 
failed.  Accordingly,  two  pots  of  seedling  Phalaris 
and  one  pot  of  seedling  Brassica  were  exposed  for  8  h. 
before  a  paraffin  lamp,  by  which  time  the  cotyledons 
of  the  former  and  the  hypocotyls  of  the  latter  were  bent 
rectangularly  towards  the  light.  The  pots  were  now 
quickly  laid  horizontally,  so  that  the  upper  parts  of 
the  cotyledons  and  of  the  hypocotyls  of  9  seedlings 
projected  vertically  upwards,  as  proved  by  a  plumb-line. 
In  this  position  they  could  not  be  acted  on  by  apo- 
geotropism, and  if  they  possessed  any  tendency  to 
straighten  themselves  or  to  bend  in  opposition  to  their 
former  heliotropic  curvature,  this  would  be  exhibited, 
for  it  would  be  opposed  at  first  very  slightly  by  apogeo- 
tropism. They  were  kept  in  the  dark  for  4  h.,  during 
which  time  they  were  twice  looked  at ;  but  no  uniform 
bending  in  opposition  to  their  former  heliotropic 
curvature  could  be  detected.  We  have  said  uniform 
bending,  because  they  circumnutated  in  their  new 
position,  and  after  2  h.  were  inclined  in  different 
directions  (between  4°  and  11°)  from  the  perpendicular. 
Their  directions  were  also  changed  after  two  additional 
hours,  and  again  on  the  following  morning.  We  may 


CHAP.  IX.  SENSITIVENESS  TO  LIGHT.  465 

therefore  conclude  that  the  bending  back  of  plants 
from  a  light,  when  this  becomes  obscure  or  is  extin- 
guished, is  wholly  due  to  apogeotropism.* 

In  our  various  experiments  we  were  often  struck 
with  the  accuracy  with  which  seedlings  pointed  to  a 
light  although  of  small  size.  To  test  this,  many  seed- 
lings of  Phalaris,  which  had  germinated  in  darkness  in 
a  very  narrow  box  several  feet  in  length,  were  placed 
in  a  darkened  room  near  to  and  in  front  of  a  lamp 
having  a  small  cylindrical  wick.  The  cotyledons  at 
the  two  ends  and  in  the  central  part  of  the  box,  would 
therefore  have  to  bend  in  widely  different  directions 
in  order  to  point  to  the  light.  After  they  had  become 
rectangularly  bent,  a  long  white  thread  was  stretched 
by  two  persons,  close  over  and  parallel,  first  to  one  and 
then  to  another  cotyledon  ;  and  the  thread  was  found 
in  almost  every  case  actually  to  intersect  the  small 
circular  wick  of  the  now  extinguished  lamp.  The 
deviation  from  accuracy  never  exceeded,  as  far  as  we 
could  judge,  a  degree  or  two.  This  extreme  accuracy 
seems  at  first  surprising,  but  is  not  really  so,  for  an 
upright  cylindrical  stem,  whatever  its  position  may 
be  with  respect  to  the  light,  would  have  exactly  half 
its  circumference  illuminated  and  half  in  shadow ;  and 
as  the  difference  in  illumination  of  the  two  sides  is 
the  exciting  cause  of  heliotropism,  a  cylinder  would 
naturally  bend  with  much  accuracy  towards  the  light. 
The  cotyledons,  however,  of  Phalaris  are  not  cylin- 
drical, but  oval  in  section ;  and  the  longer  axis  was 
to  the  shorter  axis  (in  the  one  which  was  measured) 
as  100  to  70.  Nevertheless,  no  difference  could  be 


*  It  appears  from  a  reference  heliotropically  is  at  the  same  time 

in   Wiesner   ('  Die    Undulirende  striving,  through   apogeotropism, 

Nutation  der  Internodien,'  p.  7),  to  raise  itself  into  a  vertical  posi- 

that  H.  Miiller  of  Thurgau  found  tion. 
that  a  stem    which   is    bending 


4fifi  SENSITIVENESS  TO  LIGHT.  CHAP.  IX. 

detected  in  the  accuracy  of  their  bending,  whether 
they  stood  with  their  broad  or  narrow  sides  facing 
the  light,  or  in  any  intermediate  position ;  and  so  it 
was  with  the  cotyledons  of  Avena  sativa,  which  are 
likewise  oval  in  section.  Now,  a  little  reflection  will 
show  that  in  whatever  position  the  cotyledons  may 
stand,  there  will  be  a  line  of  greatest  illumination, 
exactly  fronting  the  light,  and  on  each  side  of  this 
line  an  equal  amount  of  light  will  be  received ;  but 
if  the  oval  stands  obliquely  with  respect  to  the  light, 
this  will  be  diffused  over  a  wider  surface  on  one  side 
of  the  central  line  than  on  the  other.  We  may  there- 
fore infer  that  the  same  amount  of  light,  whether 
diffused  over  a  wider  surface  or  concentrated  on  a 
smaller  surface,  produces  exactly  the  same  effect ;  for 
the  cotyledons  in  the  long  narrow  box  stood  in  all 
sorts  of  positions  with  reference  to  the  light,  yet  all 
pointed  truly  towards  it. 

That  the  bending  of  the  cotyledons  to  the  light 
depends  on  the  illumination  of  one  whole  side  or  on 
the  obscuration  of  the  whole  opposite  side,  and  not  on 
a  narrow  longitudinal  zone  in  the  line  of  the  light 
being  affected,  was  shown  by  the  effects  of  painting 
longitudinally  with  Indian  ink  one  side  of  five  coty- 
ledons of  Phalaris.  These  were  then  placed  on  a  table 
near  to  a  south-west  window,  and  the  painted  half  was 
directed  either  to  the  right  or  left.  The  result  was  that 
instead  of  bending  in  a  direct  line  towards  the  window, 
they  were  deflected  from  the  window  and  towards  the 
unpainted  side,  by  the  following  angles,  35°,  83°,  31°, 
43°,  and  39°.  It  should  be  remarked  that  it  was  hardly 
possible  to  paint  one-half  accurately,  or  to  place  all 
the  seedlings  which  are  oval  in  section  in  quite  the 
same  position  relatively  to  the  light;  and  this  will 
account  for  the  differences  in  the  angles.  Five  coty- 


CHAP.  IX.  SENSITIVENESS   TO  LIGHT.  467 

ledons  of  A  vena  were  also  painted  in  the  same  manner, 
but  with  greater  care ;  and  they  were  laterally  de- 
flected from  the  line  of  the  window,  towards  the 
unpainted  side,  by  the  following  angles,  44°,  44°,  55°, 
51°,  and  57°.  This  deflection  of  the  cotyledons  from 
the  window  is  intelligible,  for  the  whole  unpainted 
side  must  have  received  some  light,  whereas  the  oppo- 
site and  painted  side  received  none ;  but  a  narrow 
zone  on  the  unpainted  side  directly  in  front  of  the 
window  will  have  received  most  light,  and  all  the 
hinder  parts  (half  an  oval  in  section)  less  and  less  light 
in  varying  degrees ;  and  we  may  conclude  that  the 
angle  of  deflection  is  the  resultant  of  the  action  of  the 
light  over  the  whole  of  the  unpainted  side. 

It  should  have  been  premised  that  painting  with 
Indian  ink  does  not  injure  plants,  at  least  within 
several  hours ;  and  it  could  injure  them  only  by  stop- 
ping respiration.  To  ascertain  whether  injury  was  thus 
soon  caused,  the  upper  halves  of  8  cotyledons  of  Avena 
were  thickly  coated  with  transparent  matter, — 4  with 
gum,  and  4  with  gelatine ;  they  were  placed  in  the 
morning  before  a  window,  and  by  the  evening  they 
were  normally  bowed  towards  the  light,  although  the 
coatings  now  consisted  of  dry-  crusts  of  gum  and 
gelatine.  Moreover,  if  the  seedlings  which  were  painted 
longitudinally  with  Indian  ink  had  been  injured  on 
the  painted  side,  the  opposite  side  would  have  gone 
on  growing,  and  they  would  consequently  have  become 
bowed  towards  the  painted  side ;  whereas  the  curvature 
was  always,  as  we  have  seen,  in  the  opposite  direction, 
or  towards  the  unpainted  side  which  was  exposed  to 
the  light.  We  witnessed  the  effects  of  injuring  longi- 
tudinally one  side  of  the  cotyledons  of  Avena  and 
Phalaris ;  for  before  we  knew  that  grease  was  highly 
injurious  to  them,  several  were  painted  down  one  side 


•ib'8        TRANSMITTED  EFFECTS  OF  LIGHT.       CHAP.  IX. 

with,  a  mixture  of  oil  and  lamp-black,  and  were  then 
exposed  before  a  window ;  others  similarly  treated  were 
afterwards  tried  in  darkness.  These  cotyledons  soon 
became  plainly  bowed  towards  the  blackened  side, 
evidently  owing  to  the  grease  on  this  side  having 
checked  their  growth,  whilst  growth  continued  on  the 
opposite  side.  But  it  deserves  notice  that  the  curva- 
ture differed  from  that  caused  by  light,  which  ulti- 
mately becomes  abrupt  near  the  ground.  These 
seedlings  did  not  afterwards  die,  but  were  much  injured 
and  grew  badly. 


LOCALISED  SENSITIVENESS  TO  LIGHT,  AND  ITS 
TRANSMITTED  EFFECTS. 

Phalaris  Canariensis. — Whilst  observing  the  accu- 
racy with  which  the  cotyledons  of  this  plant  became 
bent  towards  the  light  of  a  small  lamp,  we  were 
impressed  with  the  idea  that  the  uppermost  part  deter- 
mined the  direction  of  the  curvature  of  the  lower  part. 
When  the  cotyledons  are  exposed  to  a  lateral  light, 
the  upper  part  bends  first,  and  afterwards  the  bending 
gradually  extends  down  to  the  base,  and,  as  we  shall 
presently  see,  even  a  little  beneath  the  ground. 
This  holds  good  with  cotyledons  from  less  than 
•1  inch  (one  was  observed  to  act  in  this  manner  which 
was  only  *03  in  height)  to  about  '5  of  an  inch  in 
height ;  but  when  they  have  grown  to  nearly  an  inch 
in  height,  the  basal  part,  for  a  length  of  *15  to  *2  of 
an  inch  above  the  ground,  ceases  to  bend.  As  with 
young  cotyledons  the  lower  part  goes  on  bending, 
after  the  upper  part  has  become  well  arched  towards 
a  lateral  light,  the  apex  would  ultimately  point  to 
the  ground  instead  of  to  the  light,  did  not  the  upper 
part  reverse  its  curvature  and  straighten  itself,  as 


CHAP.  IX.       TRANSMITTED   EFFECTS   OF  LIGHT.        469 

soon  as  the  upper  convex  surface  of  the  bowed- 
down  portion  received  more  light  than  the  lower 
concave  surface.  The  position  ultimately  assumed  by 
young  and  upright  cotyledons,  exposed  to  light  enter- 
ing obliquely  from  above  through  a  window,  is  shown 
in  the  accompanying  figure  (Fig.  181) ;  and  here  it 
may  be  seen  that  the  whole  upper  part  has  become 
very  nearly  straight.  When  the  cotyledons  were 
exposed  before  a  bright  lamp,  standing  on  the  same 
level  with  them,  the  upper  part,  which  was  at  first 

Fig.  181. 


Phalaris  Canariensis :  cotyledons  after  exposure  in  a  box  open  on  one  side 
in  front  of  a  south-west  window  during  8  h.  Curvature  towards  the 
light  accurately  traced.  The  short  horizontal  lines  show  the  level  of 
the  ground. 

greatly  arched  towards  the  light,  became  straight  and 
strictly  parallel  with  the  surface  of  the  soil  in  the 
pots ;  the  basal  part  being  now  rectangularly  bent. 
All  this  great  amount  of  curvature,  together  with  the 
subsequent  straightening  of  the  upper  part,  was  often 
effected  in  a  .few  hours. 

After  the  uppermost  part  has  become  bowed  a  little  to  the 
light,  its  overhanging  weight  must  tend  to  increase  the  curva- 
ture of  the  lower  part ;  but  any  such  effect  was  shown  in  several 
ways  to  be  quite  insignificant.  When  little  caps  of  tin-foil 
(hereafter  to  be  described)  were  placed  on  the  summits  of  the 
cotyledons,  though  this  must  have  added  considerably  to  their 
weight,  the  rate  or  amount  of  bending  was  not  thus  increased. 
But  the  best  evidence  was  afforded  by  placing  pots  with  seedlings 
of  Phalaris  before  a  lamp  in  such  a  position,  that  the  cotyledons 
were  horizontally  extended  and  projected  at  right  angles  to  the 
line  of  light. »  In  the  course  of  5i  h.  they  were  directed  towards 
the  light  with  their  bases  bent  at  right  angles ;  and  this  abrupt 


470        TEANSMITTED   EFFECTS   OF  LIGHT.       CHAP.  IX. 

curvature  could  not  have  been  aided  in  the  least  by  the  weight 
of  the  upper  part,  which  acted  at  right  angles  to  the  plane  of 
curvature. 

It  will  be  shown  that  when  the  upper  halves  of  the  coty- 
ledons of  Phalaris  and  Avena  were  enclosed  in  little  pipes  of 
tin-foil  or  of  blackened  glass,  in  which  case  the  upper  part  was 
mechanically  prevented  from  bending,  the  lower  and  unenclosed 
part  did  not  bend  when  exposed  to  a  lateral  light;  and  it 
occurred  to  us  that  this  fact  might  be  due,  not  to  the  exclusion 
of  the  light  from  the  upper  part,  but  to  some  necessity  of  the 
bending  gradually  travelling  down  the  cotyledons,  so  that 
unless  the  upper  part  first  became  bent,  the  lower  could  not 
bend,  however  much  it  might  be  stimulated.  It  was  necessary 
for  our  purpose  to  ascertain  whether  this  notion  was  true,  and  it 
was  proved  false ;  for  the  lower  halves  of  several  cotyledons 
became  bowed  to  the  light,  although  their  upper  halves  were 
enclosed  in  little  glass  tubes  (not  blackened),  which  prevented, 
as  far  as  we  could  judge,  their  bending.  Nevertheless,  as  the 
part  within  the  tube  might  possibly  bend  a  very  little,  fine  rigid 
rods  or  flat  splinters  of  thin  glass  were  cemented  with  shellac  to 
one  side  of  the  upper  part  of  15  cotyledons ;  and  in  six  cases 
they  were  in  addition  tied  on  with  threads.  They  were  thus 
forced  to  remain  quite  straight.  The  result  was  that  the  lower 
halves  of  all  became  bowed  to  the  light,  but  generally  not  in  so 
great  a  degree  as  the  corresponding  part  of  the  free  seedlings 
in  the  same  pots ;  and  this  may  perhaps  be  accounted  for  by 
some  slight  degree  of  injury  having  been  caused  by  a  consider- 
able surface  having  been  smeared  with  shellac.  It  may  be 
added,  that  when  the  cotyledons  of  Phalaris  and  Avena  are 
acted  on  by  apogeotropism,  it  is  the  upper  part  which  begins 
first  to  bend;  and  when  this  part  was  rendered  rigid  in  the 
manner  just  described,  the  upward  curvature  of  the  basal  part 
was  not  thus  prevented. 

To  test  our  belief  that  the  upper  part  of  the  cotyledons  of 
Phalaris,  when  exposed  to  a  lateral  light,  regulates  the  bending 
of  the  lower  part,  many  experiments  were  tried ;  but  most  of  our 
first  attempts  proved  useless  from  various  causes  not  worth 
specifying.  Seven  cotyledons  had  their  tips  cut  off  for  lengths 
varying  between  '1  and  '16  of  an  inch,  and  these,  when  left 
exposed  all  day  to  a  lateral  light,  remained  upright.  In  another 
set  of  7  cotyledons,  the  tips  were  cut  off  for  a  length  of  only 
about  *05  of  an  inch  (1*27  mm.)  and  these  became  bowed  towards 


CHAP.  IX.       TKANSMITTED   EFFECTS   OF   LIGHT.        471 

a  lateral  light,  but  not  nearly  so  much  as  the  many  other  seed- 
lings in  the  same  pots.  This  latter  case  shows  that  cutting  off 
the  tips  does  not  by  itself  injure  the  plants  so  seriously  as  to 
prevent  heliotropism ;  but  we  thought  at  the  time,  that  such 
injury  might  follow  when  a  greater  length  was  cut  off,  as  in  the 
first  set  of  experiments.  Therefore,  no  more  trials  of  this  kind 
were  made,  which  we  now  regret ;  as  we  afterwards  found  that 
when  the  tips  of  three  cotyledons  were  cut  off  for  a  length  of 
•2  inch,  and  of  four  others  for  lengths  of  -14,  -12,  •!,  and  -07 
inch,  and  they  were  extended  horizontally,  the  amputation  did 
not  interfere  in  the  least  with  their  bending  vertically  upwards, 
through  the  action  of  apogeotropism,  like  unmutilated  speci- 
mens. It  is  therefore  extremely  improbable  that  the  amputation 
of  the  tips  for  lengths  of  from  '1  to  '14  inch,  could  from  the 
injury  thus  caused  have  prevented  the  lower  part  from  bending 
towards  the  light. 

We  next  tried  the  effects  of  covering  the  upper  part  of  the 
cotyledons  of  Phalaris  with  little  caps  which  were  impermeable 
to  light ;  the  whole  lower  part  being  left  fully  exposed  before  a 
south-west  window  or  a  bright  paraffin  lamp.  Some  of  the  caps 
were  made  of  extremely  thin  tin-foil  blackened  within;  these 
had  the  disadvantage  of  occasionally,  though  rarely,  being  too 
heavy,  especially  when  twice  folded.  The  basal  edges  could  be 
pressed  into  close  contact  with  the  cotyledons  ;  though  this 
again  required  care  to  prevent  injuring  them.  Nevertheless, 
any  injury  thus  caused  could  be  detected  by  removing  the  caps, 
and  trying  whether  the  cotyledons  were  then  sensitive  to  light. 
Other  caps  were  made  of  tubes  of  the  thinnest  glass,  which 
when  painted  black  served  well,  with  the  one  great  disadvantage 
that  the  lower  ends  could  not  be  closed.  But  tubes  were  used 
which  fitted  the  cotyledons  almost  closely,  and  black  paper  was 
placed,  on  the  soil  round  each,  to  check  the  upward  reflection  of 
light  from  the  soil.  Such  tubes  were  in  one  respect  far  better 
than  caps  of  tin-foil,  as  it  was  possible  to  cover  at  the  same 
time  some  cotyledons  with  transparent  and  others  with  opaque 
tubes ;  and  thus  our  experiments  could  be  controlled.  It  should 
be  kept  in  mind  that  young  cotyledons  were  selected  for  trial, 
and  that  these  when  not  interfered  with  become  bowed  down 
to  the  ground  towards  the  light. 

We  will  begin  with  the  glass-tubes.  The  summits  of  nine 
cotyledons,  differing  somewhat  in  height,  were  enclosed  for 
rather  less  than  half  their  lengths  in  uncoloured  or  transparent 
21 


472        TRANSMITTED   EFFECTS   OF   LIGHT         CHAP.  IX. 

tubes ;  and  these  were  then  exposed  before  a  south-west  window 
on  a  bright  day  for  8  h.  All  of  them  became  strongly  curved 
towards  the  light,  in  the  same  degree  as  the  many  other  free 
seedlings  in  the  same  pots ;  so  that  the  glass-tubes  certainly  did 
not  prevent  the  cotyledons  from  bending  towards  the  light. 
Nineteen  other  cotyledons  were,  at  the  same  time,  similarly 
enclosed  in  tubes  thickly  painted  with  Indian  ink.  On  five  of 
them,  the  paint,  to  our  surprise,  contracted  after  exposure 
to  the  sunlight,  and  very  narrow  cracks  were  formed,  through 
which  a  little  light  entered ;  and  these  five  cases  were  rejected. 
Of  the  remaining  14  cotyledons,  the  lower  halves  of  which  had 
been  fully  exposed  to  the  light  for  the  whole  time,  7  continued 
quite  straight  and  upright ;  1  was  considerably  bowed  to  the 
light,  and  6  were  slightly  bowed,  but  with  the  exposed  bases  of 
most  of  them  almost  or  quite  straight.  It  is  possible  that  some 
light  may  have  been  reflected  upwards  from  the  soil  and  entered 
the  bases  of  these  7  tubes,  as  the  sun  shone  brightly,  though 
bits  of  blackened  paper  had  been  placed  on  the  soil  round 
them.  Nevertheless,  the  7  cotyledons  which  were  slightly 
bowed,  together  with  the  7  upright  ones,  presented  a  most  re- 
markable contrast  in  appearance  with  the  many  other  seedlings 
in  the  same  pots  to  which  nothing  had  been  done.  The 
blackened  tubes  were  then  removed  from  10  of  these  seedlings, 
and  they  were  now  exposed  before  a  lamp  for  8  h. :  9  of  them 
became  greatly,  and  1  moderately,  curved  towards  the  light, 
proving  that  the  previous  absence  of  any  curvature  in  the 
basal  part,  or  the  presence  of  only  a  slight  degree  of  curvature 
there,  was  due  to  the  exclusion  of  light  from  the  upper  part. 

Similar  observations  were  made  on  12  younger  cotyledons 
with  their  upper  halves  enclosed  within  glass-tubes  coated  with 
black  varnish,  and  with  their  lower  halves  fully  exposed  to 
bright  sunshine.  In  these  younger  seedlings  the  sensitive  zone 
seems  to  extend  rather  lower  down,  as  was  observed  on  some 
other  occasions,  for  two  became  almost  as  much  curved  towards 
the  light  as  the  free  seedlings;  and  the  remaining  ten  were 
slightly  curved,  although  the  basal  part  of  several  of  them, 
which  normally  becomes  more  curved  than  any  other  part, 
exhibited  hardly  a  trace  of  curvature.  These  12  seedlings 
taken  together  differed  greatly  in  their  degree  of  curvature  from 
all  the  many  other  seedlings  in  the  same  pots. 

Better  evidence  of  the  efficiency  of  the  blackened  tubes  was 
incidentally  afforded  by  some  experiments  hereafter  to  be  given, 


CHAP.  IX.        TKANSMITTED   EFFECTS   OF   LIGHT.        473 

in  which  the  upper  halves  of  14  cotyledons  were  enclosed  in 
tubes  from  which  an  extremely  narrow  stripe  of  the  black 
varnish  had  been  scraped  off.  These  cleared  stripes  were 
not  directed  towards  the  window,  but  obliquely  to  one  side 
of  the  loom,  so  that  only  a  very  little  light  could  act  on  the 
upper  halves  of  the  cotyledons.  These  14  seedlings  remained 
during  eight  hours  of  exposure  before  a  south-west  window  on 
a  hazy  day  quite  upright;  whereas  all  the  other  many  free 
seedlings  in  the  same  pots  became  greatly  bowed  towards  the 
light. 

We  will  now  turn  to  the  trials  with  caps  made  of  very  thin 
tin-foil.  These  were  placed  at  different  times  on  the  summits  of 
24  cotyledons,  and  they  extended  down  for  a  length  of  between 
•15  and  '2  of  an  inch.  The  seedlings  were  exposed  to  a  lateral 
light  for  periods  varying  between  6  h.  80  m.  and  7  h.  45  m., 
which  sufficed  to  cause  all  the  other  seedlings  in  the  same  pots 
to  become  almost  rectangularly  bent  towards  the  light.  They 
varied  in  height  from  only  '04  to  T15  inch,  but  the  greater 
number  were  about  "75  inch.  Of  the  24  cotyledons  with  their 
summits  thus  protected,  3  became  much  bent,  but  not  in  the 
direction  of  the  light,  and  as  they  did  not  straighten  themselves 
through  apogeotropism  during  the  following  night,  either  the 
caps  were  too  heavy  or  the  plants  themselves  were  in  a  weak 
condition;  and  these  three  cases  may  be  excluded.  There 
are  left  for  consideration  21  cotyledons;  of  these  17  remained 
all  the  time  quite  upright ;  the  other  4  became  slightly  inclined 
to  the  light,  but  not  in  a  degree  comparable  with  that  of  the 
many  free  seedlings  in  the  same  pots.  As  the  glass-tubes,  when 
unpainted,  did  not  prevent  the  cotyledons  from  becoming 
greatly  bowed,  it  cannot  be  supposed  that  the  caps  of  very 
thin  tin-foil  did  so,  except  through  the  exclusion  of  the  light. 
To  prove  that  the  plants  had  not  been  injured,  the  caps  were 
removed  from  6  of  the  upright  seedlings,  and  these  were  exposed 
before  a  paraffin  lamp  for  the  same  length  of  time  as  before, 
and  they  now  all  became  greatly  curved  towards  the  light. 

As  caps  between  *15  and  *2  of  an  inch  in  depth  were  thus 
proved  to  be  highly  efficient  in  preventing  the  cotyledons  from 
bending  towards  the  light,  8  other  cotyledons  were  protected 
with  caps  between  only  '06  and  '12  in  depth.  Of  these,  two 
remained  vertical,  one  was  considerably  and  five  slightly  curved 
towards  the  light,  but  far  less  so  than  the  free  seedlings  in  the 
same  pots. 


474        TRANSMITTED   EFFECTS   OF  LIGHT.       CHAP.  IX. 

Another  trial  was  made  in  a  different  manner,  namely,  by 
bandaging  with  strips  of  tin-foil,  about  '2  in  breadth,  the  upper 
part,  but  not  the  actual  summit,  of  eight  moderately  young 
seedlings  a  little  over  half  an  inch  in  height.  The  summits  and 
the  basal  parts  were  thus  left  fully  exposed  to  a  lateral  light 
during  8  h. ;  an  upper  intermediate  zone  being  protected. 
With  four  of  these  seedlings  the  summits  were  exposed  for 
a  length  of  '05  inch,  and  in  two  of  them  this  part  became 
curved  towards  the  light,  but  the  whole  lower  part  remained 
quite  upright;  whereas  the  entire  length  of  the  other  two 
seedlings  became  slightly  curved  towards  the  light.  The 
summits  of  the  four  other  seedlings  were  exposed  for  a  length 
of  '04  inch,  and  of  these  one  remained  almost  upright,  whilst 
the  other  three  became  considerably  curved  towards  the  light. 
The  many  free  seedlings  in  the  same  pots  were  all  greatly 
curved  towards  the  light. 

From  these  several  sets  of  experiments,  including  those  with 
the  glass-tubes,  and  those  when  the  tips  were  cut  off,  we  may 
infer  that  the  exclusion  of  light  from  the  upper  part  of  the 
cotyledons  of  Phalaris  prevents  the  lower  part,  though  fully 
exposed  to  a  lateral  light,  from  becoming  curved.  The  summit 
for  a  length  of  -04  or  '05  of  an  inch,  though  it  is  itself  sensitive 
and  curves  towards  the  light,  has  only  a  slight  power  of  causing 
the  lower  part  to  bend.  Nor  has  the  exclusion  of  light  from  the 
summit  for  a  length  of  '1  of  an  inch  a  strong  influence  on  the 
curvature  of  the  lower  part.  On  the  other  hand,  an  exclusion 
for  a  length  of  between  '15  and  "2  of,  an  inch,  or  of  the  whole 
upper  half,  plainly  prevents  the  lower  and  fully  illuminated 
part  from  becoming  curved  in  the  manner  (see  Fig.  181)  which 
invariably  occurs  when  a  free  cotyledon  is  exposed  to  a  lateral 
light.  With  very  young  seedlings  the  sensitive  zone  seems  to 
extend  rather  lower  down  relatively  to  their  height  than  in  older 
seedlings.  We  must  therefore  conclude  that  when  seedlings 
are  freely  exposed  to  a  lateral  light  some  influence  is  trans- 
mitted from  the  upper  to  the  lower  part,  causing  the  latter  to 
bend. 

This  conclusion  is  supported  by  what  may  be  seen  to  occur 
on  a  small  scale,  especially  with  young  cotyledons,  without  any 
artificial  exclusion  of  the  light ;  for  they  bend  beneath  the  earth 
where  no  light  can  enter.  Seeds  of  Phalaris  were  covered 
with  a  layer  one-fourth  of  an  inch  in  thickness  of  very  fine 
sand,  consisting  of  extremely  minute  grains  of  silex  coated  with 


CHAP.  IX.        TRANSMITTED   EFFECTS   OF   LIGHT.        475 

oxide  of  iron.  A  layer  of  this  sand,  moistened  to  the  same 
degree  as  that  over  the  seeds,  was  spread  over  a  glass-plate ;  and 
when  the  layer  was  '05  of  an  inch  in  thickness  (carefully  mea- 
sured) no  light  from  a  bright  sky  could  be  seen  to  pass  through 
it,  unless  it  was  viewed  through  a  long  blackened  tube,  and 
then  a  trace  of  light  could  be  detected,  but  probably  much  too 
little  to  affect  any  plant.  A  layer  '1  of  an  inch  in  thickness  was 
quite  impermeable  to  light,  as  judged  by  the  eye  aided  by  the  tube. 
It  may  be  worth  adding  that  the  layer,  when  dried,  remained 
equally  impermeable  to  light.  This  sand  yielded  to  very  slight 
pressure  whilst  kept  moist,  and  in  this  state  did  not  contract 
or  crack  in  the  least.  In  a  first  trial,  cotyledons  which  had 
grown  to  a  moderate  height  were  exposed  for  8  h.  before  a  paraffin 
lamp,  and  they  became  greatly  bowed.  At  their  bases  on  the 
shaded  side  opposite  to  the  light,  well-defined,  crescentic,  open 
farrows  were  formed,  which  (measured  under  a  microscope  with 
a  micrometer)  were  from  -02  to  -03  of  an  inch  in  breadth,  and 
these  had  evidently  been  left  by  the  bending  of  the  buried  bases 
of  the  cotyledons  towards  the  light.  On  the  side  of  the  light 
the  cotyledons  were  in  close  contact  with  the  sand,  which  was  a 
very  little  heaped  up.  By  removing  with  a  sharp  knife  the 
sand  on  one  skje  of  the  cotyledons  in  the  line  of  the  light,  the 
bent  portion  and  the  open  furrows  were  found  to  extend  down 
to  a  depth  of  about  -1  of  an  inch,  where  no  light  could  enter. 
The  chords  of  the  short  buried  arcs  formed  in  four  cases  angles 
of  11°,  13°,  15°,  and  18°,  with  the  perpendicular.  By  the 
following  morning  these  short  bowed  portions  had  straightened 
themselves  through  apogeotropism. 

In  the  next  trial  much  younger  cotyledons  were  similarly 
treated,  but  were  exposed  to  a  rather  obscure  lateral  light. 
After  some  hours,  a  bowed  cotyledon,  •  3  inch  in  height,  had  an 
open  furrow  on  the  shaded  side  •  04  inch  in  breadth ;  another 
cotyledon,  only  *  13  inch  in  height,  had  left  a  furrow  *  02  inch  in 
breadth.  But  the  most  curious  case  was  that  of  a  cotyledon  which 
had  just  protruded  above  the  ground  and  was  only  '03  inch  in 
height,  and  this  was  found  to  be  bowed  in  the  direction  of  the 
light  to  a  depth  of  '2  of  an  inch  beneath  the  surface.  From 
what  we  know  of  the  impermeability  of  this  sand  to  light,  the 
upper  illuminated  part  in  these  several  cases  must  have  deter- 
mined the  curvature  of  the  lower  buried  portions.  But  an 
apparent  cause  of  doubt  may  be  suggested :  as  the  cotyledons 
are  continually  circumnutating,  they  tend  to  form  a  minute 


476        TRANSMITTED   EFFECTS   OF   LIGHT.        CHAP.  IX 

crack  or  furrow  all  round  their  bases,  which  would  admit  a 
little  light  on  all  sides ;  but  this  would  not  happen  when  they 
were  illuminated  laterally,  for  we  know  that  they  quickly  bend 
towards  a  lateral  light,  and  they  then  press  so  firmly  against  the 
sand  on  the  illuminated  side  as  to  furrow  it,  and  this  would 
effectually  exclude  light  on  this  side.  Any  light  admitted  on 
the  opposite  and  shaded  side,  where  an  open  furrow  is  formed, 
would  tend  to  counteract  the  curvature  towards  the  lamp  or 
other  source  of  the  light.  It  may  be  added,  that  the  use  of  fine 
moist  sand,  which  yields  easily  to  pressure,  was  indispensable 
in  the  above  experiments ;  for  seedlings  raised  in  common  soil, 
not  kept  especially  damp,  and  exposed  for  9  h.  30  m.  to  a  strong 
lateral  light,  did  not  form  an  open  furrow  at  their  bases  on  the 
shaded  side,  and  were  not  bowed  beneath  the  surface. 

Perhaps  the  most  striking  proof  of  the  action  of  the  upper 
on  the  lower  part  of  the  cotyledons  of  Phalaris,  when  laterally 
illuminated,  was  afforded  by  the  blackened  glass-tubes  (before 
alluded  to)  with  very  narrow  stripes  of  the  varnish  scraped 
off  on  one  side,  through  which  a  little  light  was  admitted. 
The  breadth  of  these  stripes  or  slits  varied  between  '01  and 
•02  inch  ('25  and  *51  mm.).  Cotyledons  with  their  upper 
halves  enclosed  in  such  tubes  were  placed  before  a  south-west 
window,  in  such  a  position,  that  the  scraped  stripes  did  not 
directly  face  the  window,  but  obliquely  to  one  side.  The  seed- 
lings were  left  exposed  for  8  h.,  before  the  close  of  which  time 
the  many  free  seedlings  in  the  same  pots  had  become  greatly 
bowed  towards  the  window.  Under  these  circumstances,  the 
whole  lower  halves  of  the  cotyledons,  which  had  their  summits 
enclosed  in  the  tubes,  were  fully  exposed  to  the  light  of  the 
sky,  whilst  their  upper  halves  received  exclusively  or  chiefly 
diffused  light  from  the  room,  and  this  only  through  a  very 
narrow  slit  on  one  side.  Now,  if  the  curvature  of  the  lower 
part  had  been  determined  by  the  illumination  of  this  part,  all 
the  cotyledons  assuredly  would  have  become  curved  towards 
the  window;  but  this  was  far  from  being  the  case.  Tubes 
of  the  kind  just  described  were  placed  on  several  occasions 
over  the  upper  halves  of  27  cotyledons ;  14  of  them  remained 
all  the  time  quite  vertical;  so  that  sufficient  diffused  light 
did  not  enter  through  the  narrow  slits  to  produce  any  effect 
whatever;  and  they  behaved  in  the  same  manner  as  if  their 
upper  halves  had  been  enclosed  in  completely  blackened  tubes. 
The  lower  halves  of  the  13  other  cotyledons  became  bowed 


CHAP.  IX.       TRANSMITTED  EFFECTS   OF  LIGHT.        477 

not  directly  in  the  line  of  'the  window,  but  obliquely  towards 
it ;  one  pointed  at  an  angle  of  only  18°,  but  the  remaining  12 
at  angles  varying  between  45°  and  62°  from  the  line  of  the 
window.  At  the  commencement  of  the  experiment,  pins  had 
been  laid  on  the  earth  in  the  direction  towards  which  the  slits  in 
the  varnish  faced ;  and  in  this  direction  alone  a  small  amount 
of  diffused  light  entered.  At  the  close  of  the  experiment,  7  of 
the  bowed  cotyledons  pointed  exactly  in  the  line  of  the  pins, 
and  6  of  them  in  a  line  between  that  of  the  pins  and  that  of  the 
window.  This  intermediate  position  is  intelligible,  for  any  light 
from  the  sky  which  entered  obliquely  through  the  slits  would 
be  much  more  efficient  than  the  diffused  light  which  entered 
directly  through  them.  After  the  8  h.  exposure,  the  contrast 
in  appearance  between  these  13  cotyledons  and  the  many  other 
seedlings  in  the  same  pots,  which  were  all  (excepting  the  above 
14  vertical  ones)  greatly  bowed  in  straight  and  parallel  lines 
towards  the  window,  was  extremely  remarkable.  It  is  therefore 
certain  that  a  little  weak  light  striking  the  upper  halves  of  the 
cotyledons  of  Phalaris,  is  far  more  potent  in  determining  the 
direction  of  the  curvature  of  the  lower  halves,  than  the  full 
illumination  of  the  latter  during  the  whole  time  of  exposure. 

In  confirmation  of  the  above  results,  the  effect  of  thickly 
painting  with  Indian  ink  one  side  of  the  upper  part  of  three  coty- 
ledons of  Phalaris,  for  a  length  of  •  2  inch  from  their  tips,  may  be 
worth  giving.  These  were  placed  so  that  the  unpainted  surface 
was  directed  not  towards  the  window,  but  a  little  to  one  side ; 
and  they  all  became  bent  towards  the  unpainted  side,  and  from 
the  line  of  the  window  by  angles  amounting  to  31°,  35°,  and  83°. 
The  curvature  in  this  direction  extended  down  to  their  bases, 
although  the  whole  lower  part  was  fully  exposed  to  the  light 
from  the  window. 

Finally,  although  there  can  be  no  doubt  that  the  illumination 
of  the  upper  part  of  the  cotyledons  of  Phalaris  greatly  affects 
the  power  and  manner  of  bending  of  the  lower  part,  yet  some 
observations  seemed  to  render  it  probable  that  the  simultaneous 
stimulation  of  the  lower  part  by  light  greatly  favours,  or  is 
almost  necessary,  for  its  well-marked  curvature ;  but  our  experi- 
ments were  not  conclusive,  owing  to  the  difficulty  of  excluding 
light  from  the  lower  halves  without  mechanically  preventing 
their  curvature. 

Avena  sativa. — The  cotyledons  of  this  plant  become  quickly 
bowed  towards  a  lateral  light,  exactly  like  those  of  Phalaris. 


478        TRANSMITTED   EFFECTS   OF   LIGHT.       CHAP.  IX. 

Experiments  similar  to  the  foregoing  ones  were  tried,  and  we 
will  give  the  results  as  briefly  as  possible.  They  are  somewhat 
less  conclusive  than  in  the  case  of  Phalaris,  and  this  may 
possibly  be  accounted  for  by  the  sensitive  zone  varying  in  exten- 
sion, in  a  species  so  long  cultivated  and  variable  as  the  common 
Oat.  Cotyledons  a  little  under  three-quarters  of  an  inch  in 
height  were  selected  for  trial :  six  had  their  summits  protected 
from  light  by  tin-foil  caps,  "25  inch  in  depth,  and  two  others  by 
caps  *3  inch  in  depth.  Of  these  8  cotyledons,  five  remained 
upright  during  8  hours  of  exposure,  although  their  lower  parts 
were  fully  exposed  to  the  light  all  the  time;  two  were  very  slightly, 
and  one  considerably,  bowed  towards  it.  Caps  only  *  2  or  •  22  inch 
in  depth  were  placed  over  4  other  cotyledons,  and  now  only  one 
remained  upright,  one  was  slightly,  and  two  considerably  bowed 
to  the  light.  In  this  and  the  following  cases  all  the  free  seedlings 
in  the  same  pots  became  greatly  bowed  to  the  light. 

Our  next  trial  was  made  with  short  lengths  of  thin  and 
fairly  transparent  quills ;  for  glass-tubes  of  sufficient  diameter 
to  go  over  the  cotyledons  would  have  been  too  heavy.  Firstly, 
the  summits  of  13  cotyledons  were  enclosed  in  unpainted 
quills,  and  of  these  11  became  greatly  and  2  slightly  bowed 
to  the  light ;  so  that  the  mere  act  of  enclosure  did  not  prevent 
the  lower  part  from  becoming  bowed.  Secondly,  the  summits 
of  1 1  cotyledons  were  enclosed  in  quills  •  3  inch  in  length,  painted 
so  as  to  be  impermeable  to  light;  of  these,  7  did  not  be- 
come at  all  inclined  towards  the  light,  but  3  of«  them  were 
slightly  bent  more  or  less  transversely  with  respect  to  the  line 
of  light,  and  these  might  perhaps  have  been  altogether  ex- 
cluded; one  alone  was  sfightly  bowed  towards  the  light. 
Painted  quills,  *25  inch  in  length,  were  placed  over  the  summits 
of  4  other  cotyledons ;  of  these,  one  alone  remained  upright,  a 
second  was  slightly  bowed,  and  the  two  others  as  much  bowed 
to  the  light  as  the  free  seedlings  in  the  same  pots.  These  two 
latter  cases,  considering  that  the  caps  were  '  25  in  length,  are 
inexplicable. 

Lastly,  the  summits  of  8  cotyledons  were  coated  with  flexible 
and  highly  transparent  gold-beaters'  skin,  and  all  became  as 
much  bowed  to  the  light  as  the  free  seedlings.  The  summits  of 
9  other  cotyledons  were  similarly  coated  with  gold-beaters'  skin, 
which  was  then  painted  to  a  depth  of  between  '25  and  '3  inch, 
so  as  to  be  impermeable  to  light ;  of  these  5  remained  upright, 
and  4  were  well  bowed  to  the  light,  almost  or  quite  as  well  as 


CHAP.  IX.        TRANSMITTED   EFFECTS   OF   LIGHT.         47& 

the  free  seedlings.  These  latter  four  cases,  as  well  as  the  two 
in  the  last  paragraph,  offer  a  strong  exception  to  the  rule  that 
the  illumination  of  the  upper  part  determines  the  curvature  of 
the  lower  part.  Nevertheless,  5  of  these  8  cotyledons  remained 
quite  upright,  although  their  lower  halves  were  fully  illuminated 
all  the  time ;  and  it  would  almost  be  a  prodigy  to  find  five  free 
seedlings  standing  vertically  after  an  exposure  for  several  hours 
to  a  lateral  light. 

The  cotyledons  of  Avena,  like  those  of  Phalaris,  when  growing 
in  soft,  damp,  fine  sand,  leave  an  open  crescentric  furrow  on  the 
shaded  side,  after  bending  to  a  lateral  light ;  and  they  become 
bowed  beneath  the  surface  at  a  depth  to  which,  as  we  know, 
light  cannot  penetrate.  The  arcs  of  the  chords  of  the  buried 
bowed  portions  formed  in  two  cases  angles  of  20°  and  21°  with 
the  perpendicular.  The  open  furrows  on  the  shaded  side  were, 
in  four  cases,  '008,  '016,  '024,  and  '024  of  an  inch  in  breadth. 

Brassica  oleracea  (Common  Red). — It  will  here  be  shown  that 
the  upper  half  of  the  hypocotyl  of  the  cabbage,  when  illuminated 
by  a  lateral  light,  determines  the  curvature  of  the  lower  half. 
It  is  necessary  to  experimentise  on  young  seedlings  about  half 
an  inch  or  rather  less  in  height,  for  when  grown  to  an  inch  and 
upwards  the  basal  part  ceases  to  bend.  We  first  tried  painting 
the  hypocotyls  with  Indian  ink,  or  cutting  off  their  summits  for 
various  lengths ;  but  these  experiments  are  not  worth  giving, 
though  they  confirm,  as  far  as  they  can  be  trusted,  the  results 
of  the  following  ones.  These  were  made  by  folding  gold-beaters' 
skin  once  round  the  upper  halves  of  young  hypocotyls,  and 
painting  it  thickly  with  Indian  ink  or  with  black  grease.  As 
a  control  experiment,  the  same  transparent  skin,  left  unpainted, 
was  folded  round  the  upper  halves  of  12  hypocotyls ;  and  these 
all  became  greatly  curved  to  the  light,  excepting  one,  which  was 
only  moderately  curved.  Twenty  other  young  hypocotyls  had 
the  skin  round  their  upper  halves  painted,  whilst  their  lower 
halves  were  left  quite  uncovered.  These  seedlings  were  then 
exposed,  generally  for  between  7  and  8  h.,  in  a  box  blackened 
within  and  open  in  front,  either  before  a  south-west  window  or 
a  paraffin  lamp.  This  exposure  was  amply  sufficient,  as  was 
shown  by  the  strongly-marked  heliotropism  of  all  the  free  seed- 
lings in  the  same  pots;  nevertheless,  some  were  left  exposed 
to  the  light  for  a  much  longer  time.  Of  the  20  hypocotyls 
thus  treated,  14  remained  quite  upright,  and  6  became  slightly 
bowed  to  the  light ;  but  2  of  these  latter  cases  were  not  really 


480        TRANSMITTED   EFFECTS   OF  LIGHT.       CHAP.  IX. 

exceptions,  for  on  removing  the  skin  the  paint  was  found  im- 
perfect and  was  penetrated  by  many  small  transparent  spaces 
on  the  side  which  faced  the  light.  Moreover,  in  two  other  cases 
the  painted  skin  did  not  extend  quite  halfway  down  the  hypo- 
cotyl.  Altogether  there  was  a  wonderful  contrast  in  the  several 
pots  between  these  20  hypocotvls  and  the  other  many  free 
seedlings,  which  were  all  greatly  bowed  down  to  their  bases  in 
the  direction  of  the  light,  some  being  almost  prostrate  on  the 
ground. 

The  most  successful  trial  on  any  one  day  (included  in  the 
above  results)  is  worth  describing  in  detail.  Six  young  seed- 
lings were  selected,  the  hypocotyls  of  which  were  nearly  '45  inch, 
excepting  one,  which  was  •  6  inch  in  height,  measured  from  the 
bases  of  their  petioles  to  the  ground.  Their  upper  halves, 
judged  as  accurately  as  could  be  done  by  the  eye,  were  folded 
once  round  with  gold-beaters'  skin,  and  this  was  paintec! 
thickly  with  Indian  ink.  They  were  exposed  in  an  otherwise 
darkened  room  before  a  bright  paraffin  lamp,  which  stood  on 
a  level  with  the  two  pots  containing  the  seedlings.  They 
were  first  looked  at  after  an  interval  of  5  h.  10  m.,  and  five 
of  the  protected  hypocotyla  were  found  quite  erect,  the  sixth 
being  very  slightly  inclined  to  the  light ;  whereas  all  the  many 
free  seedlings  in  the  same  two  pots  were  greatly  bowed 
to  the  light.  They  were  again  examined  after  a  continuous 
exposure  to  the  light  of  20  h.  35  m. ;  and  now  the  contrast 
between  the  two  sets  was  wonderfully  great :  for-  the  free  seed- 
lings had  their  hypocotyls  extended  almost  horizontally  in  the 
direction  of  the  light,  and  were  curved  down  to  the  ground ; 
whilst  those  with  the  upper  halves  protected  by  the  painted 
skin,  but  with  their  lower  halves  fully  exposed  to  the  light,  still 
remained  quite  upright,  with  the  exception  of  the  one  which 
retained  the  same  slight  inclination  to  the  light  which  it  had 
before.  This  latter  seedling  was  found  to  have  been  rather 
badly  painted,  for  on  the  side  facing  the  light  the  red  colour 
of  the  hypocotyl  could  be  distinguished  through  the  paint. 

We  next  tried  nine  older  seedlings,  the  hypocotyls  of  which 
varied  between  1  and  1'6  inch  in  height.  The  gold-beaters' 
skin  round  their  upper  parts  was  painted  with  black  grease  to 
a  depth  of  only  '3  inch,  that  is,  from  less  than  a  third  to  a  fourth 
or  fifth  of  their  total  heights.  They  were  exposed  to  the  light 
for  7  h.  15  m.;  and  the  result  showed  that  the  whole  of  the 
sensitive  zone,  which  determines  the  curvature  of  the  lower 


CHAP.  IX.       TRANSMITTED   EFFECTS   OF   LIGHT.        481 

part,  was  not  protected  from  the  action  of  the  light ;  for  all  9 
became  curved  towards  it,  4  of  them  very  slightly,  3  moderately, 
and  2  almost  as  much  as  the  unprotected  seedlings.  Neverthe- 
less, the  whole  9  taken  together  differed  plainly  in  their  degree 
of  curvature  from  the  many  free  seedlings,  and  from  some 
which  were  wrapped  in  unpainted  skin,  growing  in  the  same 
two  pots. 

Seeds  were  covered  with  about  a  quarter  of  an  inch  of  the  fine 
sand  described  under  Phalaris ;  and  when  the  hypocotyls  had 
grown  to  a  height  of  between  *4  and  "55  inch,  they  were  exposed 
during  9h.  before  a  paranin  lamp,  their  bases  being  at  first 
closely  surrounded  by  the  damp  sand.  They  all  became  bowed 
down  to  the  ground,  so  that  their  upper  parts  lay  near  to  and 
almost  parallel  to  the  surface  of  the  soil.  On  the  side  of  the 
light  their  bases  were  in  close  contact  with  the  sand,  which  was 
here  a  very  little  heaped  up;  on  the  opposite  or  shaded  side 
there  were  open,  crescentic  cracks  or  furrows,  rather  above  •  01 
of  an  inch  in  width ;  but  they  were  not  so  sharp  and  regular 
as  those  made  by  Phalaris  and  Avena,  and  therefore  could  not 
be  so  easily  measured  under  the  microscope.  The  hypocotyls 
were  found,  when  the  sand  was  removed  on  one  side,  to  be 
curved  to  a  depth  beneath  the  surface  in  three  cases  of  at  least 
•  1  inch,  in  a  fourth  case  of  •  11,  and  in  a  fifth  of  *  15  inch.  The 
chords  of  the  arcs  of  the  short?,  buried,  bowed  portions  formed 
angles  of  between  11°  and  15°  with  the  perpendicular.  From 
what  we  have  seen  of  the  impermeability  of  this  sand  to  light, 
the  curvature  of  the  hypocotyls  certainly  extended  down  to  a 
depth  where  no  light  could  enter;  and  the  curvature  must 
have  been  caused  by  an  influence  transmitted  from  the  upper 
illuminated  part. 

The  lower  halves  of  five  young  hypocotyls  were  surrounded  by 
unpainted  gold-beaters'  skin,  and  these,  after  an  exposure  of  8  h. 
before  a  paranin  lamp,  all  became  as  much  bowed  to  the  light 
as  the  free  seedlings.  The  lower  halves  of  10  other  young 
hypocotyls,  similarly  surrounded  with  the  skin,  were  thickly 
painted  with  Indian  ink;  their  upper  and  unprotected  halves 
became  well  curved  to  the  light,  but  their  lower  and  protected 
halves  remained  vertical  in  all  the  cases  excepting  one,  and  on 
this  the  layer  of  paint  was  imperfect.  This  result  seems  to 
prove  that  the  influence  transmitted  from  the  upper  part  is 
not  sufficient  to  cause  the  lower  part  to  bend,  unless  it  be  at 
the  same  time  illuminated ;  but  there  remains  the  doubt,  as  in 


482        TRANSMITTED   EFFECTS   OF  LIGHT.       CHAP.  IX 

the  case  of  Phalaris,  whether  the  skin  covered  with  a  rather 
thick  crust  of  dry  Indian  ink  did  not  mechanically  prevent 
their  curvature. 

Btta  vulgaris. — A  few  analogous  experiments  were  tried  on 
this  plant,  which  is  not  very  well  adapted  for  the  purpose,  as  the 
basal  part  of  the  hypocotyl,  after  it  has  grown  to  above  half  an 
inch  in  height,  does  not  bend  much  on  exposure  to  a  lateral 
light.  Four  hypocotyls  were  surrounded  close  beneath  their 
petioles  with  strips  of  thin  tin-foil,  •  2  inch  in  breadth,  and  they 
remained  upright  all  day  before  a  paraffin  lamp ;  two  others1 
were  surrounded  with  strips  '15  inch  in  breadth,  and  one  of 
these  remained  upright,  the  other  becoming  bowed ;  the  band- 
ages in  two  other  cases  were  only  •  1  inch  in  breadth,  and  both 
of  these  hypocotyls  became  bowed,  though  one  only  slightly, 
towards  the  light.  The  free  seedlings  in  the  same  pots  were 
all  fairly  well  curved  towards  the  light;  and  during  the  follow- 
ing night  became  nearly  upright.  The  pots  were  now  turned 
round  and  placed  before  a  window,  so  that  the  opposite  sides 
of  the  seedlings  were  exposed  to  the  light,  towards  which  all 
the  unprotected  hypocotyls  became  bent  in  the  course  of  7  h. 
Seven  out  of  the  8  seedlings  with  bandages  of  tin-foil  remained 
upright,  but  one  which  had  a  bandage  only  '1  inch  in  breadth, 
became  curved  to  the  light.  On  another  occasion,  the  upper 
halves  of  7  hypocotyls  were  surrounded  with  painted  gold- 
beaters' skin ;  of  these  4  remained  upright,  and  3  became  a  little 
curved  to  the  light:  at  the  same  tune  4  other  seedlings  sur- 
rounded with  unpainted  skin,  as  well  as  the  free  ones  in  the 
same  pots,  all  became  bowed  towards  the  lamp,  before  which 
they  had  been  exposed  during  22  hours. 

Radicles  of  Sinapis  alba. —  The  radicles  of  some  plants  are 
indifferent,  as  far  as  curvature  is  concerned,  to  the  action  of 
light ;  whilst  others  bend  towards  and  others  from  it.*  Whether 
these  movements  are  of  any  service  to  the  plant  is  very  doubtful, 
at  least  in  the  case  of  subterranean  roots ;  they  probably  result 
from  the  radicles  being  sensitive  to  contact,  moisture,  and  gravi- 
tation, and  as  a  consequence  to  other  irritants  which  are  never 
naturally  encountered.  The  radicles  of  Sinapis  alba,  when 
immersed  in  water  and  exposed  to  a  lateral  light,  bend  from  it, 
or  are  apheliotropic.  They  become  bent  for  a  length  of  about 
4  mm.  from  their  tips.  To  ascertain  whether  this  movement 


*  S;ichs,  'Physiologie  Ve'getale,'  1868,  p.  44. 


CHAP.  IX.       TRANSMITTED   EFFECTS   OF   LIGHT.        483 

generally  occurred,  41  radicles,  which  had  germinated  in  damp 
sawdust,  were  immersed  in  water  and  exposed  to  a  lateral  light ; 
and  they  all,  with  two  doubtful  exceptions,  became  curved  from 
the  light.  At  the  same  time  the  tips  of  54  other  radicles, 
similarly  exposed,  were  just  touched  with  nitrate  of  silver. 
They  were  blackened  for  a  length  of  from  *05  to  '07  mm.,  and 
probably  killed ;  but  it  should  be  observed  that  this  did  not 
check  materially,  if  at  all,  the  growth  of  the  upper  part;  for 
several,  which  were  measured,  increased  in  the  course  of  only 
8-9  h.  by  5  to  7  rnm.  in  length.  Of  the  54  cauterised  radicles 
one  case  was  doubtful,  25  curved  themselves  from  the  light  in 
the  normal  manner,  and  28,  or  more  than  half,  were  not  in  the 
least  apheliotropic.  There  was  a  considerable  difference,  which 
we  cannot  account  for,  in  the  results  of  the  experiments  tried 
towards  the  end  of  April  and  in  the  middle  of  September. 
Fifteen  radicles  (part  of  the  above  54)  were  cauterised  at  the 
former  period  and  were  exposed  to  sunshine,  of  which  12  failed 
to  be  apheliotropic,  2  were  still  apheliotropic,  and  1  was  doubt- 
ful. In  September,  39  cauterised  radicles  were  exposed  to  a 
northern  light,  being  kept  at  a  proper  temperature ;  and  now 
23  continued  to  be  apheliotropic  in  the  normal  manner,  and 
only  16  failed  to  bend  from  the  light.  Looking  at  the  aggregate 
results  at.  both  periods,  there  can  be  no  doubt  that  the  de- 
struction of  the  tip  for  less  than  a  millimeter  in  length  destroyed 
in  more  than  half  the  cases  their  power  of  moving  from  the 
light.  It  is  probable  that  if  the  tips  had  been  cauterised  for 
the  length  of  a  whole  millimeter,  all  signs  of  apheliotropism 
would  have  disappeared.  It  may  be  suggested  that  although 
the  application  of  caustic  does  not  stop  growth,  yet  enough  may 
be  absorbed  to  destroy  the  power  of  movement  in  the  upper 
part;  but  this  suggestion  must  be  rejected,  for  we  have  seen 
and  shall  again  see,  that  cauterising  one  side  of  the  tip  of  various 
kinds  of  radicles  actually  excites  movement.  The  conclusion 
seems  inevitable  that  sensitiveness  to  light  resides  in  the  tip 
of  the  radicle  of  Sinapis  alba;  and  that  the  tip  when  thus 
stimulated  transmits  some  influence  to  the  upper  part,  causing 
it  to  bend.  The  case  in  this  respect  is  parallel  with  that  of 
the  radicles  of  several  plants,  the  tips  of  which  are  sensitive  to 
contact  and  to  other  irritants,  and,  as  will  be  shown  in  the 
eleventh  chapter,  to  gravitation. 


484  CONCLUDING  REMAKES  AND          CHAT.  IX. 

CONCLUDING  KEMARKS  AND  SUMMARY  OF  CHAPTER. 

We  do  not  know  whether  it  is  a  general  rule  with 
seedling  plants  that  the  illumination  of  the  upper 
part  determines  the  curvature  of  the  lower  part.  But 
as  this  occurred  in  the  four  species  examined  by  us, 
belonging  to  such  distinct  families  as  the  Graminese, 
Cruciferae,  and  Chenopodeae,  it  is  probably  of  common 
occurrence.  It  can  hardly  fail  to  be  of  service  to  seed- 
lings, by  aiding  them  to  find  the  shortest  path  from 
the  buried  seed  to  the  light,  on  nearly  the  same 
principle  that  the  eyes  of  most  of  the  lower  crawling 
animals  are  seated  at  the  anterior  ends  of  their  bodies. 
It  is  extremely  doubtful  whether  with  fully  developed 
plants  the  illumination  of  one  part  ever  affects  the 
curvature  of  another  part.  The  summits  of  5  young 
plants  of  Asparagus  officinalis  (varying  in  height  be- 
tween 1*1  and  2*7  inches,  and  consisting  of  several 
short  internodes)  were  covered  with  caps  of  tin-foil 
from  0*3  to  0*35  inch  in  depth;  and  the  lower  un- 
covered parts  became  as  much  curved  towards  a  lateral 
light,  as  were  the  free  seedlings  in  the  same  pots. 
Other  seedlings  of  the  same  plant  had  their  summits 
painted  with  Indian  ink  with  the  same  negative  result. 
Pieces  of  blackened  paper  were  gummed  to  the  edges 
and  over  the  blades  of  some  leaves  on  young  plants  of 
Tropseolum  majus  and  Ranunculus  ficaria ;  these  were 
then  placed  in  a  box  before  a  window,  and  the  petioles 
of  the  protected  leaves  became  curved  towards  the 
light,  as  much  as  those  of  the  unprotected  leaves. 

The  foregoing  cases  with  respect  to  seedling  plants 
have  been  fully  described,  not  only  because  the  trans- 
mission of  any  effect  from  light  is  a  new  physiological 
fact,  but  because  we  think  it  tends  to  modify  somewhat 
the  current  views  on  heliotropic  movements.  Until 


CHAP.  IX.  SUMMARY  OF   CHAPTER.  485 

lately  such  movements  were  believed  to  result  simply 
from  increased  growth  on  the  shaded  side.  At  present 
it  is  commonly  admitted  *  that  diminished  light  in- 
creases the  turgescence  of  the  cells,  or  the  extensibility 
of  the  cell-walls,  or  of  both  together,  on  the  shaded 
side,  and  that  this  is  followed  by  increased  growth. 
But  Pfeffer  has  shown  that  a  difference  in  the  tur- 
gescence on  the  two  sides  of  a  pulvinus, — that  is,  an 
aggregate  of  small  cells  which  have  ceased  to  grow  at 
an  early  age, — is  excited  by  a  difference  in  the  amount 
of  light  received  by  the  two  sides;  and  that  move- 
ment is  thus  caused  without  being  followed  by  in- 
creased growth  on  the  more  turgescent  side.t  All 
observers  apparently  believe  that  light  acts  directly 
on  the  part  which  bends,  but  we  have  seen  with  the 
above  described  seedlings  that  this  is  not  the  case. 
Their  lower  halves  were  brightly  illuminated  for  hours, 
and  yet  did  not  bend  in  the  least  towards  the  light, 
though  this  is  the  part  which  under  ordinary  circum- 
stances bends  the  most.  It  is  a  still  more  striking 
fact,  that  the  faint  illumination  of  a  narrow  stripe  on 
one  side  of  the  upper  part  of  the  cotyledons  of  Phalaris 
determined  the  direction  of  the  curvature  of  the  lower 
part ;  so  that  this  latter  part  did  not  bend  towards  the 
bright  light  by  which  it  had  been  fully  illuminated, 


*  Emil  GodlewsM  has  given  63,  123,  &c.  Frank  has  also 

('Bot.  Zeitung,'  1879,  Nos.  6-9)  insisted  ('Die  Naturliche  wa- 

an  excellent  account  (p.  120)  of  gerechte  Riclitung  von  Pflan- 

the  present  state  of  the  question.  zentheilen,'  1870,  p.  53)  on  the 

See  also  Vines  in  'Arbeiten  des  important  part  which  the  pulvini 

Bot.  Inst.  in  Wiirzburg,'  1878,  B.  of  the  leaflets  of  compound  leaves 

ii.  pp.  114-147.  Hugo  de  Vries  play  in  placing  the  leaflets  in  a 

has  recently  published  a  still  proper  position  with  respect  to  the 

more  important  article  on  this  light.  This  holds  good,  especially 

subject : 4  Bot.  Zeitung,'  Dec.  19th.  with  the  leaves  of  climbing  plants, 

and  26th,  1879.  which  are  carried  into  all  sorts 

t  '  Die  Periodischen  Bewegun-  of  positions,  ill-adapted  for  the 

gen  der  Blattoryane,'  1875,  pp.  7,  action  of  the  light. 


486  CONCLUDING   REMAKKS  AND          CHAP.  IX. 

but  obliquely  towards  one  side  where  only  a  little 
light  entered.  These  results  seem  to  imply  the  pre- 
sence of  some  matter  in  the  upper  part  which  is  acted 
on  by  light,  and  which  transmits  its  effects  to*  the 
lower  part.  It  has  been  shown  that  this  transmission 
is  independent  of  the  bending  of  the  upper  sensitive 
part.  We  have  an  analogous  case  of  transmission  in 
Drosera,  for  when  a  gland  is  irritated,  the  basal  and 
not  the  upper  or  intermediate  part  of  the  tentacle 
bends.  The  flexible  and  sensitive  filament  of  Diomea 
likewise  transmits  a  stimulus,  without  itself  bending ; 
as  does  the  stem  of  Mimosa. 

Light  exerts  a  powerful  influence  on  most  vege- 
table tissues,  and  there  can  be  no  doubt  that  it 
generally  tends  to  check  their  growth.  But  when  the 
two  sides  of  a  plant  are  illuminated  in  a  slightly 
different  degree,  it  does  not  necessarily  follow  that 
the  bending  towards  the  illuminated  side  is  caused  by 
changes  in  the  tissues  of  the  same  nature  as  those 
which  lead  to  increased  growth  in  darkness.  We 
know  at  least  that  a  part  may  bend  from  the  light, 
and  yet  its  growth  may  not  be  favoured  by  light. 
This  is  the  case  with  the  radicles  of  Sinapis  alba,  which 
are  plainly  apheliotropic ;  nevertheless,  they  grow 
quicker  in  darkness  than  in  light.*  So  it  is  with 
many  aerial  roots,  according  to  Wiesner  ;f  but  there 
are  other  opposed  cases.  It  appears,  therefore,  that 
light  does  not  determine  the  growth  of  apheliotropic 
parts  in  any  uniform  manner. 

We  should  bear  in  mind  that  the  power  of  bending 
to  the  light  is  highly  beneficial  to  most  plants.  There 


*  Francis    Darwin,  '  Uber  das  Heft  iii.,  1880,  p.  521. 

Wachsthum    negativ    heliutropi-  f  '  Sitzb.  derk.  Akad.  der  Wis- 

scher   Wurzeln ' :    4  Arbeiten  des  sensch '  (Vienna),  1880,  p.  12. 
Bot.  lust,   in  Wurzburg,'  B.  ii., 


CHAP.  IX.  SUMMARY   OF  CHAPTER.  487 

is  therefore  no  improbability  in  this  power  having  been 
specially  acquired.  In  several  respects  light  seems  to 
act  on  plants  in  nearly  the  same  manner  as  it  does 
on  animals  by  means  of  the  nervous  system.*  With 
seedlings  the  effect,  as  we  have  just  seen,  is  trans- 
mitted from  one  part  to  another.  An  animal  may  be 
excited  to  move  by  a  very  small  amount  of  light ;  and 
it  has  been  shown  that  a  difference  in  the  illumination 
of  the  two  sides  of  the  cotyledons  of  Phalaris,  which 
could  not  be  distinguished  by  the  human  eye,  sufficed 
to  cause  them  to  bend.  It  has  also  been  shown  that 
there  is  no  close  parallelism  between  the  amount  of 
light  which  acts  on  a  plant  and  its  degree  of  curva- 
ture; it  was  indeed  hardly  possible  to  perceive  any 
difference  in  the  curvature  of  some  seedlings  of  Phalaris 
exposed  to  a  light,  which,  though  dim,  was  very  much 
brighter  than  that  to  which  others  had  been  exposed. 
The  retina,  after  being  stimulated  by  a  bright  light, 
feels  the  effect  for  some  time ;  and  Phalaris  continued 
to  bend  for  nearly  half  an  hour  towards  the  side  which 
had  been  illuminated.  The  retina  cannot  perceive 
a  dim  light  after  it  has  been  exposed  to  a  bright  one ; 
and  plants  which  had  been  kept  in  the  daylight 
during  the  previous  day  and  morning,  did  not  move 
so  soon  towards  an  obscure  lateral  light  as  did  others 
which  had  been  kept  in  complete  darkness. 

Even  if  light  does  act  in  such  a  manner  on  the 
growing  parts  of  plants  as  always  to  excite  in  them 
a  tendency  to  bend  towards  the  more  illuminated 
side — a  supposition  contradicted  by  the  foregoing 
experiments  on  seedlings  and  by  all  apheliotropic 


*  Sachs  has  made  some  striking  See  his  paper  *  V?ber  orthotrope 

remarks  to  the  same  effect  with,  und   plagiotrope  Pflanzentheile,' 

respect    to    the    various    stimuli  *  Arb.  des.  Bot.  Inst.  in  Wiirzburg,' 

which  excite  movement  in  plants.  ]  879  B.  ii.  p.  282. 


488  CONCLUDING   REMARKS  AND          CHAP.  IX. 

organs — yet  the  tendency  differs  greatly  in  different 
species,  and  is  variable  in  degree  in  the  individuals  of 
the  same  species,  as  may  be  seen  in  almost  any  pot 
of  seedlings  of  a  long  cultivated  plant.*  There  is 
therefore  a  basis  for  the  modification  of  this  tendency 
to  almost  any  beneficial  extent.  That  it  has  been 
modified,  we  see  in  many  cases :  thus,  it  is  of  more 
importance  for  insectivorous  plants  to  place  their 
leaves  in  the  best  position  for  catching  insects  than 
to  turn  their  leaves  to  the  light,  and  they  have 
no  such  power.  If  the  stems  of  twining  plants  were 
to  bend  towards  the  light,  they  would  often  be  drawn 
away  from  their  supports ;  and  as  we  have  seen  they 
do  not  thus  bend.  As  the  steins  of  most  other  plants 
are  heliotropic,  we  may  feel  almost  sure  that  twining 
plants,  which  are  distributed  throughout  the  whole 
vascular  series,  have  lost  a  power  that  their  non- 
climbing  progenitors  possessed.  Moreover,  with  Ipo- 
moea,  and  probably  all  other  twiners,  the  stem  of  the 
young  plant,  before  it  begins  to  twine,  is  highly  helio- 
tropic, evidently  in  order  to  expose  the  cotyledons  or 
the  first  true  leaves  fully  to  the  light.  With  the  Ivy  the 
steins  of  seedlings  are  moderately  heliotropic,  whilst 
those  of  the  same  plants  when  grown  a  little  older 


*  Strnsburger  has  shown  in  his  the  light.  Some  individuals,  more- 
interesting  work  ('Wirkung  des  over,  appear  to  be  indifferent  to 
Lichtes  .  .  .  auf  Schwarmsporen,'  the  light;  and  those  of  different 
1878),  that  the  movement  of  the  species  behave  very  differently, 
swarm-spores  of  various  lowly  The  brighter  the  light,  the 
organised  plants  to  a  lateral  light  straighter  is  their  course.  They 
is  influenced  by  their  stage  of  exhibit  also  for  a  short  time  the 
development,  by  the  temperature  after-effects  of  light.  In  all  these 
to  which  they  are  subjected,  by  respects  they  re-emble  the  higher 
the  degree  of  illumination  under  plants.  See,  also,  Stahl,  *  Ueber 
which  they  have  been  raised,  and  den  einfluss  der  Lichts  auf  die 
by  other  unknown  causes;  so  that  Bewegungs -  erscheinungeu  der 
the  swarm-spores  of  the  same  Schwarmsporen '  Verh.  d.  phys.- 
species  may  move  across  the  field  roed.  Geselsshalft  in  Wiirzburg, 
of  the  microscope  either  to  or  from  B.  xii.  1878. 


CHAP.  IX.  SUMMARY   OF   CHAPTER.  489 

are  apheliotropic.  Some  tendrils  which  consist  of 
modified  leaves — organs  in  all  ordinary  cases  strongly 
cliahelio tropic— have  been  rendered  apheliotropic,  and 
their  tips  crawl  into  any  dark  crevice. 

Even  in  the  case  of  ordinary  heliotropic  movements, 
it  is  hardly  credible  that  they  result  directly  from 
the  action  of  the  light,  without  any  special  adaptation. 
We  may  illustrate  what  we  mean  by  the  hygroscopic 
movements  of  plants  :  if  the  tissues  on  one  side  of  an 
organ  permit  of  rapid  evaporation,  they  will  dry 
quickly  and  contract,  causing  the  part  to  bend  to  this 
side.  Now  the  wonderfully  complex  movements  of 
the  pollinia  of  Orchis  pyramidalis,  by  which  they  clasp 
the  proboscis  of  a  moth  and  afterwards  change  their 
position  for  the  sake  of  depositing  the  pollen-masses 
on  the  double  stigma — or  again  the  twisting  move- 
ments, by  which  certain  seeds  bury  themselves  in 
the  ground* — follow  from  the  manner  of  drying  of 
the  parts  in  question  ;  yet  no  one  will  suppose  that 
these  results  have  been  gained  without  special  adapta- 
tion. Similarly,  we  are  led  to  believe  in  adaptation 
when  we  see  the  hypocotyl  of  a  seedling,  which  contains 
chlorophyll,  bending  to  the  light ;  for  although  it  thus 
receives  less  light,  being  now  shaded  by  its  own  coty- 
ledons, it  places  them — the  more  important  organs — in 
the  best  position  to  be  fully  illuminated.  The  hypo- 
cotyl may  therefore  be  said  to  sacrifice  itself  for  the 
good  of  the  cotyledons,  or  rather  of  the  whole  plant. 
But  if  it  be  prevented  from  bending,  as  must  some- 
times occur  with  seedlings  springing  up  in  an  en- 
tangled mass  of  vegetation,  the  cotyledons  themselves 
bend  so  as  to  face  the  light ;  the  one  farthest  off  rising 


*  Francis  Darwin, '  On  the  Hy-      actions  Linn.  Soc.,' series  ii.  vol.  i. 
groscopic  Mechanism/  &c.,  *  Trans-      p.  149,  1876. 


490  CONCLUDING  EEMARKS  AND          CHAP.  IX. 

up,  and  that  nearest  to  the  light  sinking  down,  or 
both  twisting  laterally.*  We  may,  also,  suspect  that 
the  extreme  sensitiveness  to  light  of  the  upper  part 
of  the  sheath-like  cotyledons  of  the  Gramineae,  and 
their  power  of  transmitting  its  effects  to  the  lower 
part,  are  specialised  arrangements  for  finding  the 
shortest  path  to  the  light.  With  plants  growing  on 
a  bank,  or  thrown  prostrate  by  the  wind,  the  manner 
in  which  the  leaves  move,  even  rotating  on  their  own 
axes,  so  that  their  upper  surfaces  may  be  again  directed 
to  the  light,  is  a  striking  phenomenon.  Such  facts 
are  rendered  more  striking  when  we  remember  that 
too  intense  a  light  injures  the  chlorophyll,  and  that 
the  leaflets  of  several  Leguminosaa  when  thus  exposed 
bend  upwards  and  present  their  edges  to  the  sun,  thus 
escaping  injury.  On  the  other  hand,  the  leaflets  of 
Averrhoa  and  Oxalis,  when  similarly  exposed,  bend 
downwards. 

It  was  shown  in  the  last  chapter  that  heliotropism 
is  a  modified  form  of  circumnutation ;  and  as  every 
growing  part  of  every  plant  circumnutates  more  or  less, 
we  can  understand  how  it  is  that  the  power  of  bending 
to  the  light  has  been  acquired  by  such  a  multitude 
of  plants  throughout  the  vegetable  kingdom.  The 
manner  in  which  a  circumnutating  movement — that 
is,  one  consisting  of  a  succession  of  irregular  ellipses 
or  loops — is  gradually  converted  into  a  rectilinear 
course  towards  the  light,  has  been  already  explained. 
First,  we  have  a  succession  of  ellipses  with  their 
longer  axes  directed  towards  the  light,  each  of  which 


*  Wiesner  has  made  remarks  to  tracted   from    B.   Ixxvii.   (1878). 

nearly  the  same  effect  with  respect  Sitb.  der  k.  Akad.  der  Wisseusch, 

to  leaves :  '  Die  undulirende  Nu-  Wien. 
tation  der  Internodien,'  p.  6,  ex- 


CHAP.  IX.  SUMMARY   OF   CHAPTER  491 

is  described  nearer  and  nearer  to  its  source ;  then  the 
loops  are  drawn  out  into  a  strongly  pronounced  zigzag 
line,  with  here  and  there  a  small  loop  still  formed. 
At  the  same  time  that  the  movement  towards  the  light 
is  increased  in  extent  and  accelerated,  that  in  the 
opposite  direction  is  lessened  and  retarded,  and  at  last 
stopped.  The  zigzag  movement  to  either  side  is 
likewise  gradually  lessened,  so  that  finally  the  course 
becomes  rectilinear.  Thus  under  the  stimulus  of  a 
fairly  bright  light  there  is  no  useless  expenditure  of 
force. 

As  with  plants  every  character  is  more  or  less 
variable,  there  seems  to  be  no  great  difficulty  in  be- 
lieving that  their  circumnutating  movements  may 
have  been  increased  or  modified  in  any  beneficial 
manner  by  the  preservation  of  varying  individuals. 
The  inheritance  of  habitual  movements  is  a  necessary 
contingent  for  this  process  of  selection,  or  the  survival 
of  the  fittest ;  and  we  have  seen  good  reason  to  believe 
that  habitual  movements  are  inherited  by  plants.  In 
the  case  of  twining  species  the  circumnutating  move- 
ments have  been  increased  in  amplitude  and  rendered 
more  circular ;  the  stimulus  being  here  an  internal 
or  innate  one.  With  sleeping  plants  the  movements 
have  been  increased  in  amplitude  and  often  changed 
in  direction ;  and  here  the  stimulus  is  the  alternation 
of  light  and  darkness,  aided,  however,  by  inheritance. 
In  the  case  of  heliotropism,  the  stimulus  is  the  unequal 
illumination  of  the  two  sides  of  the  plant,  and  this 
determines,  as  in  the  foregoing  cases,  the  modifica- 
tion of  the  circumnutating  movement  in  such  a  manner 
that  the  organ  bends  to  the  light.  A  plant  which 
has  been  rendered  heliotropic  by  the  above  means, 
might  readily  lose  this  tendency,  judging  from  the 
cases  already  given,  as  soon  as  it  became  useless  or 


492  CONCLUDING  REMARKS.  CHAP.  IX. 

injurious.  A  species  which  has  ceased  to  be  helio- 
tropic  might  also  be  rendered  apheliotropic  by  the 
preservation  of  the  individuals  which  tended  to  cir- 
cumnutate  (though  the  cause  of  this  and  most  other 
variations  is  unknown)  in  a  direction  more  or  less 
opposed  to  that  whence  the  light  proceeded.  In  like 
manner  a  plant  might  be  rendered  diaheliotropic. 


CHAP.  X     MOVEMENTS  EXCITED  BY  GRAVITATION.    493 


CHAPTER  X. 

MODIFIED  CIRCUMNUTATION  :  MOVEMENTS  EXCITED  BY  GRAVITATION. 

Means  of  observation  — ApogeotropLm  —  Cytisus — Verbena — Beta — 
Gradual  conversion  of  the  movement  of  circumnutation  into  apogeo- 
tropism  in  Rubus,  Lilium,  Phalaris,  Avena,  and  Bra.-sica — Apogeo- 
tropism  retarded  by  heliotropism — Effected  by  the  aid  of  joints 
or  pulvini — Movements  of  flower-peduncles  of  Oxalis — General 
remarks  on  apogeotropism — Geotropism — Movements  of  radicles — 
Burying  of  seed-capsules — Use  of  process — Trifolium  subterraneum 
— Arachis — Amphicarpsea — Diageotropism — Conclusion. 

OUK  object  in  the  present  chapter  is  to  show  that 
geotropism,  apogeotropism,  and  diageotropism  are  mo- 
dified forms  of  circumnutation.  Extremely  fine  fila- 
ments of  glass,  bearing  two  minute  triangles  of  paper, 
were  fixed  to  the  summits  of  young  stems,  frequently 
to  the  hypocotyls  of  seedlings,  to  flower-peduncles, 
radicles,  &c.,  and  the  movements  of  the  parts  were 
then  traced  in  the  manner  already  described  on 
vertical  and  horizontal  glass-plates.  It  should  be 
remembered  that  as  the  stems  or  other  parts  become 
more  and  more  oblique  with  respect  to  the  glasses,  the 
figures  traced  on  them  necessarily  become  more  and 
more  magnified.  The  plants  were  protected  from  light, 
excepting  whilst  each  observation  was  being  made,  and 
then  the  light,  which  was  always  a  dim  one,  was 
allowed  to  enter  so  as  to  interfere  as  little  as  possible 
with  the  movement  in  progress ;  and  we  did  not  detect 
any  evidence  of  such  interference. 

When  observing  the  gradations  between  circumnu- 


494  MODIFIED   CIRCUMNUTATION.  CHAP.  X. 

tation  and  lieliotropism,  we  had  the  great  advantage  of 
being  able  to  lessen  the  light ;  but  with  geotropism 
analogous  experiments  were  of  course  impossible. 
We  could,  however,  observe  the  movements  of  stems 
placed  at  first  only  a  little  from  the  perpendicular,  in 
which  case  geotropism  did  not  act  with  nearly  so  much 
power,  as  when  the  stems  were  horizontal  and  at  right 
angles  to  the  force.  Plants,  also,  were  selected  which 
were  but  feebly  geotropic  or  apogeotropic,  or  had 
become  so  from  having  grown  rather  old.  Another 
plan  was  to  place  the  stems  at  first  so  that  they  pointed 
30  or  40  degrees  beneath  the  horizon,  and  then  apo- 
geotropism  had  a  great  amount  of  work  to  do  before 
the  stem  was  rendered  upright  ;  and  in  this  case 
ordinary  circumnutation  was  often  not  wholly  oblite- 
rated. Another  plan  was  to  observe  in  the  evening 
plants  which  during  the  day  had  become  greatly 
curved  heliotropically ;  for  their  stems  under  the  gra- 
dually waning  light  very  slowly  became  upright  through 
the  action  of  apogeotropism ;  and  in  this  case  modified 
circumnutation  was  sometimes  well  displayed. 

Apogeotropism.— Plants  were  selected  for  observation  almost 
by  chance,  excepting  that  they  were  taken  from  widely  different 
families.  If  the  stem  of  a  plant  which  is  even  moderately 
sensitive  to  apogeotropism  be  placed  horizontally,  the  upper 
growing  part  bends  quickly  upwards,  so  as  to  become  perpen- 
dicular; and  the  line  traced  by  joining  the  dots  successively 
made  on  a  glass-plate,  is  generally  almost  straight.  For  in- 
stance, a  young  Cytisusfragrans,  12  inches  in  height,  was  placed 
so  that  the  stem  projected  10°  beneath  the  horizon,  and  its 
course  was  traced  during  72  h.  At  first  it  bent  a  very  little 
downwards  (Fig.  182),  owing  no  doubt  to  the  weight  of  the 
stem,  as  this  occurred  with  most  of  the  other  plants  observed, 
though,  as  they  were  of  course  circumnutating,  the  short  down- 
ward lines  were  often  oblique.  After  three-quarters  of  an  hour 
the  stem  began  to  curve  upwards,  quickly  during  the  first  two 
hours,  but  much  more  slowly  during  the  afternoon  and  night, 


CHAP.  X. 


APOGEOTROPISM. 


495 


and  on  the  following  day.     During  the  second  night  it  fell 

a     little,    and     circumnutated 

during  the  following  day;  but  it 

also  moved  a  short  distance  to 

the  right,  which  was  caused  by 

a  little  light    having  been  ac- 
cidentally admitted  on  this  side. 

The    stem    was    now    inclined 

60°  above  the  horizon,  and  had 

therefore  risen  70°.    With  time 

allowed  it  would  probably  have 

become  upright,  and  no   doubt 

would  have   continued  circum- 

nutating.     The  sole  remarkable 

feature  in  the  figure  here  given 

is  the  straightness  of  the  course 

pursued.     The   stem,  however, 

did  not  move   upwards   at  an 

equable  rate,  and  it  sometimes 

stood    almost    or     quite    still. 

Such  periods  probably  represent 
attempts  to  circumnutate  in  a 
direction    opposite    to    apogees 
tropism. 
The    herbaceous    stem   of   a 

Verbena  melindres  (?)  laid  hori- 
zontally, rose  in  7  h.  so  much 
that  dt  could  no  longer  be 
observed  on  the  vertical  glass 
which  stood  in  front  of  the  plant. 
The  long  line  which  was  traced 
was  almost  absolutely  straight. 
After  the  7  h.  it  still  continued 
to  rise,  but  now  circumnutated 
slightly.  On  the  following  day 


it  stood  upright,  and  circum- 
nutated regularly,  as  shown  in 
Fig.  82,  given  in  the  fourth 
chapter.  The  stems  of  several 
other  plants  which  were  highly 
sensitive  to  apogeotropism  rose 
up  in  almost  straight  lines,  and 
22 


ment  of  stem  from  10°  beneath  to 
60°  above  horizon,  traced  on  ver- 
tical glass,  from  8.30  A.M.  March 
12th  to  10.30  P.M.  13th.  The  sub- 
sequent circumnutating  movement 
is  likewise  shown  up  to  6.45  A.M. 
on  the  15th.  Nocturnal  course 
represented,  as  usual,  by  a  broken 
line.  Movement  not  greatly  mag- 
nified, and  tracing  reduced  to  two- 
thirds  of  original  scale. 


496 


MODIFIED   CIRCUMXUTATION,  CHAP.  X. 


then  suddenly  began  to  circumnutate. 
Fig.  183. 


Beta  vulgaris:  apogaotropic  movement 
of  hypocotyl  from  19°  beneath  horizon 
to  a  vertical  position,  with  subsequent 
circumuutation,  traced  on  a  vertical 
and  on  a  horizontal  glass-plate,  from 
8.28  A.M.  Sept.  28th  to  8.40  A.M.  29th. 
Figure  reduced  to  one-third  of  original 
scale. 


A  partially  etiolated 
and  somewhat  old  hypocotyl 
of  a  seedling  cabbage  (21 
inches  in  height)  was  so 
sensitive  that  when  placed 
at  an  angle  of  only  23°  from 
the  perpendicular,  it  became 
vertical  in  33  minutes.  As 
it  could  not  have  been 
strongly  acted  upon  by 
apogeotropism  in  the  above 
slightly  inclined  position, 
we  expected  that  it  would 
have  circumnutated,  or  at 
least  have  moved  in  a  zig- 
zag course.  Accordingly, 
dots  were  made  every  3 
minutes;  but,  when  these 
were  joined,  the  line  was 
nearly  straight.  After  this 
hypocotyl  had  become  up- 
right it  still  moved  onwards 
for  half  an  hour  in  the  same 
general  direction,  but  in  a 
zigzag  manner.  During  the 
succeeding  9  h.  it  circum- 
nutated regularly,  and  de- 
scribed 3  large  ellipses.  In 
this  case  apogeotropism, 
although  acting  at  a  very 
unfavourable  angle,  quite 
overcame  the  ordinary  cir- 
cumnutating  movement. 

The  hypocotyls  of  Beta 
vulgaris  are  highly  sensitive 
to  apogeotropism.  One  was 
placed  so  as  to  project  19° 
beneath  the  horizon ;  it  fell 
at  first  a  very  little  (see 
Fig.  183),  no  doubt  owing 
to  its  weight ;  but  as  it  was 
circumnutating  the  line  was 


CHAP.  X.  APOGEOTROPISM.  497 

oblique  During  the  next  3  h.  8  m.  it  rose  in  a  nearly  straight 
line,  passing  through  an  angle  of  109°,  and  then  (at  12.8  P.M.) 
stood  upright.  It  continued  for  55  m.  to  move  in  the  same 
general  direction  beyond  the  perpendicular,  but  in  a  zigzag 
course.  It  returned  also  in  a  zigzag  line,  and  then  circumnu- 
tated  regularly,  describing  three  large  ellipses  during  the 
remainder  of  the  day.  It  should  be  observed  that  the  ellipses 
in  this  figure  are  exaggerated  in  size,  relatively  to  the  length  of 
the  upward  straight  line,  owing  to  the  position  of  the  vertical 
and  horizontal  glass-plates.  Another  and  somewhat  old  hypo- 
cotyl  was  placed  so  as  to  stand  at  only  31°  from  the  perpen- 
dicular, in  which  position  apogeotropism  acted  on  it  with  little 
force,  and  its  course  accordingly  was  slightly  zigzag. 

The  sbeath-like  cotyledons  of  P/talaris  Canariensis  are  ex- 
tremely sensitive  to  apogeotropism.  One  was  placed  so  as  to 
project  40°  beneath  the  horizon.  Although  it  was  rather  old 
and  1'3  inch  in  height,  it  became  vertical  in  4  h.  30  m.,  having 
passed  through  an  angle  of  130°  in  a  nearly  straight  line.  It  then 
suddenly  began  to  circumnutate  in  the  ordinary  manner.  The 
cotyledons  of  this  plant,  after  the  first  leaf  has  begun  to  pro- 
trude, are  but  slightly  apogeotropic,  though  they  still  continue 
to  circumnutate.  One  at  this  stage  of  development  was  placed 
horizontally,  and  did  not  become  upright  even  after  13  h.,  and  its 
course  was  slightly  zigzag.  So,  again,  a  rather  old  hypocotyl 
of  Cassia  tora  (1£  inch  in  height)  required  28  h.  to  become  up- 
right, and  its  course  was  distinctly  zigzag ;  whilst  younger  hypo- 
cotyls  moved  much  more  quickly  and  in  a  nearly  straight  line. 

When  a  horizontally  placed  stem  or  other  organ  rises  in  a 
zigzag  line,  we  may  infer  from  the  many  cases  given  in  our 
previous  chapters,  that  we  have  a  modified  form  of  circumnu- 
tation ;  but  when  the  course  is  straight,  there  is  no  evidence 
of  circumnutation,  and  any  one  might  maintain  that  this  latter 
movement  had  been  replaced  by  one  of  a  wholly  distinct  kind. 
This  view  seems  the  more  probable  when  (as  sometimes 
occurred  with  the  hypocotyls  of  Brassica  and  Beta,  the  stems  of 
Cucurbita,  and  the  cotyledons  of  Phalaris)  the  part  in  question 
after  bending  up  in  a  straight  course,  suddenly  begins  to  circum- 
nutate to  the  full  extent  and  in  the  usual  manner.  A  fairly 
good  instance  of  a  sudden  change  of  this  kind — that  is*,  from  a 
nearly  straight  upward  movement  to  one  of  circumnutation — 
is  shown  in  Fig.  183 ;  but  more  striking  instances  were  occa- 
sionally observed  with  Beta,  Brassica,  and  Phalaris. 

We    will   now  describe   a   few  cases  in  which  it  may  be 


498  MODIFIED   CIKCUMNUTATION.  CHAP.  X 

seen  how  gradually  circumnutation  becomes  changed  into  apogeo- 
tropism, under  circumstances  to  be  specified 
in  each  instance. 

Rubus  idceus  (hybrid).— A  young  plant,  11 
\  inches  in  height,  growing  in  a  pot,  was  placed 

J  horizontally;  and  the  upward  movement  was 
-g,  traced  during  nearly  70  h. ;  but  the  plant, 
though  growing  vigorously,  was  not  highly 
*&  ~j  sensitive  to  apogeotropism,  or  it  was  not 
§^  capable  of  quick  movement,  for  during  the 
|>  a  .  above  time  it  rose  only  67°.  We  may  see  in 
2  -|f  the  diagram  (Fig.  184)  that  during  the  first 
**  «  day  of  12  h.  it  rose  in  a  nearly  straight  line. 
§^  When  placed  horizontally,  it  was  evidently 
"2^  circumnutating,  for  it  rose  at  first  a  little, 
J^Ji  notwithstanding  the  weight  of  the  stem,  and 
•3  §  then  sank  down ;  so  that  it  did  not  start  on 
•Jo  its  permanently  upward  course  until  1  h. - 
£  -a  25  m.  had  elapsed.  On  the  second  day,  by 
J  S  which  time  it  had  risen  considerably,  and 
^  1  when  apogeotropism  acted  on  it  with  somewhat 
|  2  less  power-,  its  course  during  15  3  h.  was  clearly 
%  .*?  zigzag,  and  the  rate  of  the  upward  movement 
B*  was  not  equable.  During  the  third  day,  also 
»  •£  of  15 2  h.,  when  apogeotropism  acted  on  it 
°  °]  with  still  less  power,  the  stem  plainly  circum- 
S  ^.  nutated,  for  it  moved  during  this  day  3  times 
£  oo  up  and  3  times  down,  4  times  to  the  left  and 
|  2  4  to  the  right.  But  the  course  was  so  complex 
2  •£  that  it  could  hardly  be  traced  on  the  glass. 
2  "•  We  can,  however,  see  that  the  successively 
8  2  formed  irregular  ellipses  rose  higher  and 
g^  higher.  Apogeotropism  continued  to  act  on 
*  a  the  fourth  morning,  as  the  stem  was  still 
^^  rising,  though  it  now  stood  only  23°  from  the 
perpendicular.  In  this  diagram  the  several 
&~*  stages  may  be  followed  by  which  an  almost 

J         rectilinear,  upward,  apogeotropic  course  first 
becomes   zigzag,  and    then    changes   into  a 
J         circumnutating  movement,  with  most  of  the 
t§          successively  formed,  irregular  ellipses  directed 

upwards. 
LUium  auratum. — A  plant  23  inches  in  height  was  placed 


CHAP.  X 


APOGEOTROPISM. 


499 


Fig.  185. 


horizontally,  and  the  upper  part  of  the  stem  rose  58°  in  46  h., 
in  the  manner  shown  in  the  accom- 
panying diagram  (Fig.  185).  "We  here 
see  that  during  the  whole  of  the 
second  day  of  152  h.,  the  stem  plainly 
circumnutated  whilst  bending  upwards 
through  apogeotropism.  It  had  still 
to  rise  considerably,  for  when  the  last 
dot  in  the  figure  was  made,  it  stood 
32°  from  an  upright  position. 

Plialaris  Canariensis. — A  cotyledon 
of  this  plant  (1*3  inch  in  height)  has 
already  been  described  as  rising  in 
4  h.  30  m.  from  40°  beneath  the  hori- 
zon into  a  vertical  position,  passing 
through  an  angle  of  130°  in  a  nearly 
straight  line,  and  then  abruptly  be- 
ginning to  circumnutate.  Another 
somewhat  old  cotyledon  of  the  same 
height  (but  from  which  a  true  leaf 
had  not  yet  protruded),  was  similarly 
placed  at  40°  beneath  the  horizon.  For 
the  first  4  h.  it  rose  in  a  nearly  straight 
course  (Fig.  186),  so  that  by  1.10  P.M. 
it  was  highly  inclined,  and  now  apo- 
geotropism acted  on  it  with  much  less 
power  than  before,  and  it  began  to 
zigzag.  At  4.15  P.M.  (i.e.  in  7  h.  from 
the  commencement)  it  stood  vertically, 
and  afterwards  continued  to  circum- 
nutate in  the  usual  manner  about  the 
same  spot.  Here  then  we  have  a 
graduated  change  from  a  straight  up- 
ward apogeotropic  course  into  circum-  Lilmm  auratum  :  apogeo- 
nutation,  instead  of  an  abrupt  change,  tropic  movement  of  stem, 
as  in  the  former  case. 

Avena  saliva. — The  sheath-like  coty- 
ledons, whilst  young,  are  strongly  apo- 
geotropic ;  and  some  which  were  placed 
at  45°  beneath  the  horizon  rose  90°  in 
7  or  8  h.  in  lines  almost  absolutely 
straight.  An  oldish  cotyledon,  from  which  the  first  leaf  began  to 


traced  on  a  vertical  glass 
during  2  days  and  2 
nights,  from  10.40  A.M. 
March  18th  to  8  A.M. 
20th.  Figure  reduced  to 
one-half  of  the  original 
scale. 


500 


MODIFIED  CIRCU3INUTATION. 


CHAP.  X. 


Fig.  186. 


ffJO'am.  \ 


Phalanx  Canariensis :  apogcotropic  move- 
ment of  cotyledon,  traced  on  a  vertical 
and  horizontal  glass,  from  9. 10  A.M.  Sept. 
19th  to  9  A  M.  20th.  Figure  here  re- 
duced to  one-fifth  of  original  scale. 


protrude  whilst  the  fol- 
lowing observations  were 
being  made,  was  placed 
at  10°  beneath  the  horizon, 
and  it  rose  only  59°  in 
24  h.  It  behaved  rather 
differently  from  any  other 
plant,  observed  by  us,  for 
during  the  first  4^  h.  it 
rose  in  a  line  not  far  from 
straight ;  during  the  next 
62  h.  it  circimmutated, 
that  is,  it  descended  and 
again  ascended  in  a 
strongly  marked  zigzag 
course;  it  then  resumed 
its  upward  movement  in 
a  moderately  straight  line, 
and,  with  time  allowed, 
no  doubt  would  have  be- 
come upright.  In  this 
case,  after  the  first  4£  h., 
ordinary  circumnutation 
almost  completely  con- 
quered for  a  time  apogeo- 
tropism. 

Brassica  oleracea. — The 
hypocotyls  of  several 
young  seedlings  placed 
horizontally,  rose  up  ver- 
tically in  the  course  of  6 
or  7  h.  in  nearly  straight 
lines.  A  seedling  which 
had  grown  in  darkness  to 
a  height  of  2i  inches,  and 
was  therefore  rather  old 
and  not  highly  sensitive, 
was  placed  so  that  the 
hypocotyl  projected  at  be- 
tween 30°  and  40°  beneath 
the  horizon.  The  upper 
part  alone  became  curved 


CHAP.  X. 


APOGEOTROPISM. 


501 


Fig.  187. 


upwards,  and  rose  during  the  first  3  h.  10  m.  in  a  nearly  straight 
line  (Fig.  187);  but  it  was  not 
possible  to  trace  the  upward  move- 
ment on  the  vertical  glass  for  the 
first  1  h.  10  m.,  so  that  the  nearly 
straight  line  in  the  diagram  ought 
to  have  been  much  longer.  During 
the  next  11  h.  the  hypocotyl  circum- 
nutated,  describing  irregular  figures, 
each  of  which  rose  a  little  above 
the  one  previously  formed.  During 
the  night  and  following  early  morn- 
ing it  continued  to  rise  in  a  zigzag 
course,  so  that  apogeotropism  was 
still  acting.  At  the  close  of  our  ob- 
servations, after  23  h.  (represented 
by  the  highest  dot  in  the  diagram) 
the  hypocotyl  was  still  32°  from 
the  perpendicular.  There  can  be 
little  doubt  that  it  would  ulti- 
mately have  become  upright  by 
describing  an  additional  number 
of  irregular  ellipses,  one  above  the 
other. 

Apogeotropism  retarded  l>y  Helio- 
tropism.  —  When  the  stem  of  any 
plant  bends  during  the  day  towards 
a  lateral  light,  the  movement  is 
opposed  by  apogeotropism ;  but  as 
the  light  gradually  wanes  in  the 
evening  the  latter  power  slowly 
gains  the  upper  hand,  and  draws  Bmssica  okracea:  apogeotropic 
the  stem  back*  into  a  vertical  movementof hypocotyl, traced 
position.  Here  then  we  have  a 
good  opportunity  for  observing  how 
apogeotropism  acts  when  very 
nearly  balanced  by  an  opposing 
force.  For  instance,  the  plumule 
of  Tropceolum  majus  (see  former 
Fig.  175)  moved  towards  the  dim 
evening  light  in  a  slightly  zigzag 
line  until  6.45  P.M.,  it  then  returned  on  its  course  until 


on  vertical  glass,  from  9.20 
A.M.  Sept.  12th  to  8.30  A.M. 
13th.  The  upper  part  of  the 
figure  is  more  magnified  than 
the  lower  part.  If  the  whole 
course  had  been  traced,  the 
straight  upright  line  would 
have  been  much  longer.  Figure 
here  reduced  to  one-third  of 
the  original  scale. 


502  MODIFIED   CIRCUMNUTATION.  CHAP.  X 

10.40  P.M.,  during  which  time  it  zigzagged  and  described  an 
ellipse  of  considerable  size.  The  hypocotyl  of  Brassica  okracea 
(see  former  Fig.  173)  moved  in  a  straight  line  to  the  light  until 
5.15  P.M.,  and  then  from  the  light,  making  in  its  backward 
course  a  great  rectangular  bend,  and  then  returned  for  a  short 
distance  towards  the  former  source  of  the  light ;  no  observa- 
tions were  made  after  7.10  P.M.,  but  during  the  night  it  re- 
covered its  vertical  position.  A  hypocotyl  of  Cassia  tora  moved 
in  the  evening  in  a  somewhat  zigzag  line  towards  the  failing 
light  until  6  P.M.,  and  was  now  bowed  20°  from  the  perpendi- 
cular ;  it  then  returned  on  its  course,  making  before  10.30  P.M. 
four  great,  nearly  rectangular  bends  and  almost  completing  an 
ellipse.  Several  other  analogous  cases  were  casually  observed, 
and  in  all  of  them  the  apogeotropic  movement  could  be  seen  to 
consist  of  modified  circumnutation. 

Apogeotropic  Movements  effected  by  the  aid  of  joints  or  pulvini. 
— Movements  of  this  kind  are  well  known  to  occur  in  the 
Graminese,  and  are  effected  by  means  of  the  thickened  bases 
of  their  sheathing  leaves;  the  stem  within  being  in  this  part 
thinner  than  elsewhere.*  According  to  the  analogy  of  all  other 
pulvini,  such  joints  ought  to  continue  circumnutating  for  a 
long  period,  after  the  adjoining  parts  have  ceased  to  grow.  We 
therefore  wished  to  ascertain  whether  this  was  the  case  with 
the  Graminese ;  for  if  so,  the  upward  curvature  of  their  stemsj 
when  extended  horizontally  or  laid  prostrate,  would  be  explained 
in  accordance  with  our  view — namely,  that  apogeotropism 
results  from  modified  circumnutation.  After  these  joints  have 
curved  upwards,  they  are  fixed  in  their  new  position  by  increased 
growth  along  their  lower  sides. 

Lolium  perenne. — A  young  stem,  7  inches  in  height,  consist- 
ing of  3  internodes,  with  the  flower-head  not  yet  protruded, 
was  selected  for  observation.  A  long  and  very  thin  glass  fila- 
ment was  cemented  horizontally  to  the  stem  'close  above  the 
second  joint,  3  inches  above  the  ground.  This  joint  was  subse- 
quently proved  to  be  in  an  active  condition,  as  its  lower  side 
swelled  much  through  the  action  of  apogeotropism  (in  the 
manner  described  by  De  Vries)  after  the  haulm  had  been 
fastened  down  for  24  h.  in  a  horizontal  position.  The  pot  was 


*  This  structure  has  been  re-  die  Aufrichtung  des  gelagerten 
cently  described  by  De  Vries  in  Getreides,'  in  *  Landwirthschaft- 
an  interesting  article,  *  Ueber  Hche  Jahrbiicher,'  1880,  p.  473. 


CHAP.  X.  APOGEOTKOPISM.  503 

so  placed  that  the  end  of  the  filament  stood  beneath  the  2-inch 
object  glass  of  a  microscope  with  an  eye-piece  micrometer,  each 
division  of  which  equalled  ^^  of  an  inch.  The  end  of  the  fila- 
ment was  repeatedly  observed  during  6  h.,  and  was  seen  to  be 
in  constant  movement ;  and  it  crossed  5  divisions  of  the  micro- 
meter (y^  inch)  in  2  h.  Occasionally  it  moved  forwards  by 
jerks,  some  of  which  were  y^^  inch  in  length,  and  then  slowly 
retreated  a  little,  afterwards  again  jerking  forwards.  These 
oscillations  were  exactly  like  those  described  under  Brassica 
and  Dionaea,  but  they  occurred  only  occasionally.  We  may 
therefore  conclude  that  this  moderately  old  joint  was  continually 
circumnutating  on  a  small  scale. 

Alopecurus  pratensis. — A  young  plant,  11  inches  in  height,  with 
the  flower-head  protruded,  but  with  the  florets  not  yet  expanded, 
had  a  glass  filament  fixed  close  above  the  second  joint,  at  a 
height  of  only  2  inches  above  the  ground.  The  basal  internode, 
2  inches  in  length,  was  cemented  to  a  stick  to  prevent  any 
possibility  of  its  circumnutating.  The  extremity  of  the  filament, 
which  projected  about  50°  above  the  horizon,  was  often  observed 
during  24  h.  in  the  same  manner  as  in  the  last  case.  Whenever 
looked  at,  it  was  always  in  movement,  and  it  crossed  30  divisions 
of  the  micrometer  (-/$  inch)  in  85  h. ;  but  it  sometimes  moved 
at  a  quicker  rate,  for  at  one  time  it  crossed  5  divisions  in  l£  h. 
The  pot  had  to  be  moved  occasionally,  as  the  end  of  the  filament 
travelled  beyond  the  field  of  vision ;  but  as  far  as  we  could 
judge  it  followed  during  the  daytime  a  semicircular  course ; 
and  it  certainly  travelled  in  two  different  directions  at  right 
angles  to  one  another.  It  sometimes  oscillated  in  the  same 
manner  as  in  the  last  species,  some  of  the  jerks  forwards  being 
as  much  as  y^o  of  an  inch.  We  may  therefore  conclude  that 
the  joints  in  this  and  the  last  species  of  grass  long  continue  to 
circumnutate ;  so  that  this  movement  would  be  ready  to  be 
converted  into  an  apogeotropic  movement,  whenever  the  stem 
was  placed  in  an  inclined  or  horizontal  position. 

Movements  of  the  Flower -peduncles  of  Oxalis  carnosa,  due  to 
apogeotropism  and  other  forces. — The  movements  of  the  main 
peduncle,  and  of  the  three  or  four  sub-peduncles  which  each 
main  peduncle  of  this  plant  bears,  are  extremely  complex,  and 
are  determined  by  several  distinct  causes.  Whilst  the  flowers 
are  expanded,  both  kinds  of  peduncles  circumnutate  about  the 
same  spot,  as  we  have  seen  (Fig  91)  in  the  fourth  chapter. 
But  soon  after  the  flowers  have  begun  to  wither  the  sub- 


504  MODIFIED   CIRCUMNUTATION.  CHAP.  X. 

peduncles  bend  downwards,  and  this  is  due  to  epinasty;  for 
on  two  occasions  when  pots  were  laid  horizontally,  the  sub- 
peduncles  assumed  the  same  position  relatively  to  the  main 
peduncle,  as  would  have  been  the  case  if  they  had  remained 
upright;  that  is,  each  of  them  formed  with  it  an  angle  of 
about  40°.  If  they  had  been  acted  on  by  geotropism  or  aphelio- 
tropism  (for  the  plant  was  illuminated  from  above),  they  would 
have  directed  themselves  to  the  centre  of  the  earth.  A  main 
peduncle  was  secured  to  a  stick  in  an  upright  position,  and  one 
of  the  upright  sub-peduncles  which  had  been  observed  circum- 
nutating  whilst  the  flower  was  expanded,  continued  to  do  so  for 
at  least  24  h.  after  it  had  withered.  It  then  began  to  bend 
downwards,  and  after  36  h.  pointed  a  little  beneath  the  horizon. 
A  new  figure  was  now  begun  (A,  Fig.  188),  and  the  sub-peduncle 
was  traced  descending  in  a  zigzag  line  from  7.20  P.M.  on  the  19th 
to  9  A.M.  on  the  22nd.  It  now  pointed  almost  perpendicularly 
downwards,  and  the  glass  filament  had  to  be  removed  and 
fastened  transversely  across  the  base  of  the  young  capsule. 
We  expected  that  the  sub-peduncle  would  have  been  motionless 
in  its  new  position ;  but  it  continued  slowly  to  swing,  like  a 
pendulum,  from  side  to  side,  that  is,  in  a  plane  at  right  angles 
to  that  in  which  it  had  descended.  This  circumnutating  move- 
ment was  observed  from  9  A.M.  on  22nd  to  9  A.M.  24th,  as  shown 
at  B  in  the  diagram.  We  were  not  able  to  observe  this  par- 
ticular sub-peduncle  any  longer;  but  it  would  certainly  have 
gone  on  circumnutating  until  the  capsule  was  nearly  ripe  (which 
requires  only  a  short  time),  and  it  would  then  have  moved 
upwards. 

The  upward  movement  (C,  Fig.  188)  is  effected  in  part  by  the 
whole  sub-peduncle  rising  in  the  same  manner  as  it  had  pre- 
viously descended  through  epinasty — namely,  at  the  joint  where 
united  to  the  main  peduncle.  As  this  upward  movement 
occurred  with  plants  kept  in  the  dark  and  in  whatever  position 
the  main  peduncle  was  fastened,  it  could  not  have  been  caused 
by  heliotropism  or  apogeotropism,  but  by  hyponasty.  Besides 
this  movement  at  the  joint,  there  is  another  of  a  very  different 
kind,  for  the  sub-peduncle  becomes  upwardly  bent  in  the  middle 
part.  If  the  sub-peduncle  happens  at  the  time  to  be  inclined 
much  downwards,  the  upward  curvature  is  so  great  that  the 
whole  forms  a  hook.  The  upper  end  bearing  the  capsule,  thus 
always  places  itself  upright,  and  as  this  occurs  in  darkness,  and 
in  whatever  position  the  main  peduncle  may  have  been  secured, 


CHAP.  X. 


APOGEOTKOPISM. 


505 


the  upward  curvature  cannot  be  due  to  heliotropism  or  hypo- 
nasty,  but  to  apogeotropism. 

9  Fig.  188. 


"--... 


Oxalis  camosa  :  movements  of  flower-peduncle,  traced  on  a  vertical  glass  : 
A,  epinastic  downward  movement ;  B,  circumnutation  whilst  depend- 
ing vertically  ;  C,  subsequent  upward  movement,  due  to  apogeotropism 
and  hyponasty  combined 


506  MODIFIED   CIECUMNUTATION.  CHAP.  X. 

In  order  to  trace  this  upward  movement,  a  filament  was  fixed 
to  a  sub-peduncle  bearing  a  capsule  nearly  ripe,  which  was 
beginning  to  bend  upwards  by  the  two  means  just  described.  Its 
course  was  traced  (see  C,  Fig.  188)  during  53  h.,  by  which  time 
it  had  become  nearly  upright.  The  course  is  seen  to  be  strongly 
zigzag;  together  with  some  little  loops.  We  may  therefore  con- 
clude that  the  movement  consists  of  modified  circumnutation. 

The  several  species  of  Oxalis  probably  profit  in  the  following 
manner  by  their  sub-peduncles  first  bending  downwards  and 
then  upwards.  They  are  known  to  scatter  their  seeds  by  the 
bursting  of  the  capsule;  the  walls  of  which  are  so  extremely 
thin,  like  silver  paper,  that  they  would  easily  be  permeated  by 
rain.  But  as  soon  as  the  petals  wither,  the  sepals  rise  up  and 
enclose  the  young  capsule,  forming  a  perfect  roof  over  it  as 
soon  as  the  sub-peduncle  has  bent  itself  downwards.  By  its 
subsequent  upward  movement,  the  capsule  stands  when  ripe 
at  a  greater  height  above  the  ground  by  twice  the  length  of  the 
sub-peduncle,  than  it  did  when  dependent,  and  is  thus  able 
to  scatter  its  seeds  to  a  greater  distance.  The  sepals,  which 
enclose  the  ovarium  whilst  it  is  young,  present  an  additional 
adaptation  by  expanding  widely  when  the  seeds  are  ripe,  so  as 
not  to  interfere  with  their  dispersal.  In  the  case  of  Oxalis 
acetosella,  the  capsules  are  said  sometimes  to  bury  themselves 
under  loose  leaves  or  moss  on  the  ground,  but  this  cannot  occur 
with  those  of  0.  carnosa,  as  the  woody  stem  is  too  high. 

Oxalis  acetosella. — The  peduncles  are  furnished  with  a  joint  in 
Fig.-189. 


Oxalis  acetosella  :  course  pursued  by  the  upper  part  of  a  peduncle,  whilst 
rising,  traced  from  1 1  A.M.  June  1  st  to  9  A.M.  3rd.  Figure  here  re- 
duced to  one-half  of  the  original  scale. 

the  middle,  so  that  the  lower  part  answers  to  the  main  peduncle, 


CHAP.  X.  APOGEOTKOPISM.  507 

and  the  upper  part  to  one  of  the  sub-peduncles  of  0.  carnosa. 
The  upper  part  bends  downwards,  after  the  flower  has  begun 
to  wither,  and  the  whole  peduncle  then  forms  a  hook;  that 
this  bending  is  due  to  epinasty  we  may  infer  from  the  case  of 
0.  carnosa.  When  the  pod  is  nearly  ripe,  the  upper  part 
straightens  itself  and  becomes  erect ;  and  this  is  due  to  hypo- 
nasty  or  apogeotropism,  or  both  combined,  and  not  to  helio- 
tropism,  for  it  occurred  in  darkness.  The  short,  hooked  part  of 
the  peduncle  of  a  cleistogamic  flower,  bearing  a  pod  nearly  ripe, 
was  observed  in  the  dark  during  three  days.  The  apex  of  the 
pod  at  first  pointed  perpendicularly  down,  but  in  the  course  of 
three  days  rose  90°,  so  that  it  now  projected  horizontally.  The 
course  during  the  two  latter  days  is  shown  in  Fig.  189;  and 
it  may  be  seen  how  greatly  the  peduncle,  whilst  rising,  cifcum- 
nutated.  The  lines  of  chief  movement  were  at  right  angles 
to  the  plane  of  the  originally  hooked  part.  The  tracing  was 
not  continued  any  longer ;  but  after  two  additional  days,  the 
peduncle'  with  its  capsule  had  become  straight  and  stood 
upright. 

Concluding  Remarks  on  Apogeotropism. — When  apo- 
geotropism is  rendered  by  any  means  feeble,  it  acts, 
as  shown  in  the  several  foregoing  cases,  by  increasing 
the  always  present  circumnutating  movement  in  a 
direction  opposed  to  gravity,  and  by  diminishing  that 
in  the  direction  of  gravity,  as  well  as  that  to  either 
side.  The  upward  movement  thus  becomes  unequal 
in  rate,  and  is  sometimes  interrupted  by  stationary 
periods.  Whenever  irregular  ellipses  or  loops  are  still 
formed,  their  longer  axes  are  almost  always  directed 
in  the  line  of  gravity,  in  an  analogous  manner  as 
occurred  with  heliotropic  movements  in  reference  to 
the  light.  As  apogeotropism  acts  more  and  more 
energetically,  ellipses  or  loops  cease  to  be  formed,  and 
the  course  becomes  at  first  strongly,  and  then  less  and 
less  zigzag,  and  finally  rectilinear.  From  this  grada- 
tion in  the  nature  of  the  movement,  and  more  especially 
from  all  growing  parts,  which  alone  (except  when  pul- 
vini  are  present)  are  acted  on  by  apogeotropism,  con- 


508  MODIFIED   CIRCUMNUTATION.  CHAP.  X. 

tinually  circumnutating,  we  may  conclude  that  even 
a  rectilinear  course  is  merely  an  extremely  modified 
form  of  circumnutation.  It  is  remarkable  that  a  stem 
or  other  organ  which  is  highly  sensitive  to  apogeo- 
tropism,  and  which  has  bowed  itself  rapidly  upwards 
in  a  straight  line,  is  often  carried  beyond  the  vertical, 
as  if  by  momentum.  It  then  bends  a  little  backwards 
to  a  point  round  which  it  finally  circumnutates.  Two 
instances  of  this  were  observed  with  the  hypocotyls  of 
Beta  vulgaris,  one  of  which  is  shown  in  Fig.  183,  and 
two  other  instances  with  the  hypocotyls  of  Brassica. 
This  momentum-like  movement  probably  results  from 
the  accumulated  effects  of  apogeotropism.  For  the 
sake  of  observing  how  long  such  after-effects  lasted, 
a  pot  with  seedlings  of  Beta  was  laid  on  its  side  in  the 
dark,  and  the  hypocotyls  in  3  h.  15m.  became  highly 
inclined.  The  pot,  still  in  the  dark,  was  then  placed 
upright,  and  the  movements  of  the  two  hypocotyls  were 
traced ;  one  continued  to  bend  in  its  former  direction, 
now  in  opposition  to  apogeotropism,  for  about  37  m., 
perhaps  for  48  m. ;  but  after  61  m.  it  moved  in  an 
opposite  direction.  The  other  hypocotyl  continued 
to  move  in  its  former  course,  after  being  placed 
upright,  for  at  least  37  m. 

Different  species  and  different  parts  of  the  same 
species  are  acted  on  by  apogeotropism  in  very  dif- 
ferent degrees.  Young  seedlings,  most  of  which  cir- 
cumnutate  quickly  and  largely,  bend  upwards  and 
become  vertical  in  much  less  time  than  do  any  older 
plants  observed  by  us;  but  whether  this  is  due  to 
their  greater  sensitiveness  to  apogeotropism,  or  merely 
to  their  greater  flexibility  we  do  not  know.  A  hypo- 
cotyl of  Beta  traversed  an  angle  of  109°  in  3  h.  8  m., 
and  a  cotyledon  of  Phalaris  an  angle  of  130°  in  4  h. 
30  m.  On  the  other  hand,  the  stem  of  a  herbaceous 


CHAP.  X.  APOGEOTROPISM.  509 

Verbena  rose  90°  in  about  24  h. ;  that  of  Bubus  67°, 
in  70  h. ;  that  of  Cytisus  70°,  in  72  h. ;  that  of  a  young 
American  Oak  only  37°,  in  72  h.  The  stem  of  a 
young  Cyperus  alternifolius  rose  only  11°  in  96  h. ; 
the  bending  being  confined  to  near  its  base.  Though 
the  sheath-like  cotyledons  of  Phalaris  are  so  extremely 
sensitive  to  apogeotropism,  the  first  true  leaves  which 
protrude  from  them  exhibited  only  a  trace  of  this 
action.  Two  fronds  of  a  fern,  Nephrodium  molle,  both 
of  them  young  and  one  with  the  tip  still  inwardly 
curled,  were  kept  in  a  horizontal  position  for  46  h., 
and  during  this  time  they  rose  so  little  that  it  was 
doubtful  whether  there  was  any  true  apogeotropic 
movement. 

The  most  curious  case  known  to  us  of  a  difference 
in  sensitiveness  to  gravitation,  and  consequently  of 
movement,  in  different  parts  of  the  same  organ,  is  that 
offered  by  the  petioles  of  the  cotyledons  of  Ipomoea 
lepiophylla.  The  basal  part  for  a  short  length  where 
united  to  the  undeveloped  hypocotyl  and  radicle  is 
strongly  geotropic,  whilst  the  whole  upper  part  is 
strongly  apogeotropic.  But  a  portion  near  the  blades 
of  the  cotyledons  is  after  a  time  acted  on  by  epinasty 
and  curves  downwards,  for  the  sake  of  emerging  in  the 
form  of  an  arch  from  the  ground;  it  subsequently 
straightens  itself,  and  is  then  again  acted  on  by  apo- 
geotropism. 

A  branch  of  Gucurbita  ovifera,  placed  horizontally, 
moved  upwards  during  7  h.  in  a  straight  line,  until  it 
stood  at  40°  above  the  horizon ;  it  then  began  to  cir- 
cumnutate,  as  if  owing  to  its  trailing  nature  it  had  no 
tendency  to  rise  any  higher.  Another  upright  branch 
was  secured  to  a  stick,  close  to  the  base  of  a  tendril, 
and  the  pot  was  then  laid  horizontally  in  the  dark. 
In  this  position  the  tendril  circumnutated  and  made 


510  MODIFIED   CIRCUMNUTATIOX.  CHAP.  X. 

several  large  ellipses  during  14  h.,  as  it  likewise  did 
on  the  following  day ;  but  during  this  whole  time  it 
was  not  in  the  least  affected  by  apogeotropism.  On  the 
other  hand,  when  branches  of  another  Cucurbitaceous 
plant,  Ecliinocytis  lolata,  were  fixed  in  the  dark  so  that 
the  tendrils  depended  beneath  the  horizon,  these  began 
immediately  to  bend  upwards,  and  whilst  thus  moving 
they  ceased  to  circumnutate  in  any  plain  manner; 
but  as  soon  as  they  had  become  horizontal  they  re- 
commenced to  revolve  conspicuously.*  The  tendrils 
of  Passiflora  gracilis  are  likewise  apogeotropic.  Two 
branches  were  tied  down  so  that  their  tendrils  pointed 
many  degrees  beneath  the  horizon.  One  was  observed 
for  8  h.,  during  which  time  it  rose,  describing  two 
circles,  one  above  the  other.  The  other  tendril  rose 
in  a  moderately  straight  line  during  the  first  4  h., 
making  however  one  small  loop  in  its  course ;  it  then 
stood  at  about  45°  above  the  horizon,  where  it  circum- 
nutated  during  the  remaining  8  h.  of  observation. 

A  part  or  organ  which  whilst  young  is  extremely 
sensitive  to  apogeotropism  ceases  to  be  so  as  it  grows 
old ;  and  it  is  remarkable*  as  showing  the  independence 
of  this  sensitiveness  and  of  the  circumnutating  move- 
ment, that  the  latter  sometimes  continues  for  a  time 
after  all  power  of  bending  from  the  centre  of  the  earth 
has  been  lost.  Thus  a  seedling  Orange  bearing  only 
3  young  leaves,  with  a  rather  stiff  stem,  did  not  curve 
in  the  least  upwards  during  24  h.  whilst  extended 
horizontally ;  yet  it  circumnutated  all  the  time  over 
a  small  space.  The  hypocotyl  of  a  young  seedling 
of  Cassia  tora,  similarly  placed,  became  vertical  in 
12  h. ;  that  of  an  older  seedling,  1 J  inch  in  height, 


*  For  details  see  '  The  Movements  and  Habits  of  Climbing  Plants,' 
1875,  p.  131. 


CHAP.  X.  APOGEOTROPISM.  511 

became  so  in  28  h. ;  and  that  of  another  still  older 
one,  1J  inch  in  height,  remained  horizontal  during 
two  days,  but  distinctly  circumnutated  during  this 
whole  time. 

When  the  cotyledons  of  Phalaris  or  Avena  are  laid 
horizontally,  the  uppermost  part  first  bends  upwards, 
and  then  the  lower  part ;  consequently,  after  the  lower 
part  has  become  much  curved  upwards,  the  upper  part 
is  compelled  to  curve  backwards  in  an  opposite  direc- 
tion, in  order  to  straighten  itself  and  to  stand  ver- 
tically ;  and  this  subsequent  straightening  process  is 
likewise  due  to  apogeotropism.  The  upper  part  of 
8  young  cotyledons  of  Phalaris  were  made  rigid  by 
being  cemented  to  thin  glass  rods,  so  that  this  part 
could  not  bend  in  the  least ;  nevertheless,  the  basal 
part  was  not  prevented  from  curving  upward.  A  stem 
or  other  organ  which  bends  upwards  through  apogeo- 
tropism exerts  considerable  force;  its  own  weight, 
which  has  of  course  to  be  lifted,  was  sufficient  in 
almost  every  instance  to  cause  the  part  at  first  to  bend 
a  little  downwards ;  but  the  downward  course  was 
often  rendered  oblique  by  the  simultaneous  circum- 
nutating  movement.  The  cotyledons  of  Avena  placed 
horizontally,  besides  lifting  their  own  weight,  were 
able  to  furrow  the  soft  sand  above  them,  so  as  to  leave 
little  crescentic  open  spaces  on  the  lower  sides  of  their 
bases;  and  this  is  a  remarkable  proof  of  the  force 
exerted. 

As  the  tips  of  the  cotyledons  of  Phalaris  and  Avena 
bend  upwards  through  the  action  of  apogeotropism 
before  the  basal  part,  and  as  these  same  tips  when 
excited  by  a  lateral  light  transmit  some  influence  to 
the  lower  part,  causing  it  to  bend,  we  thought  that 
the  same  rule  might  hold  good  with  apogeotropism. 
Consequently,  the  tips  of  7  cotyledons  of  Phalaris  were 


512  MODIFIED  CIRCUMNUTATION.  CHAP.  X 

cut  off  for  a  length  in  three  cases  of  '2  inch  and  in 
the  four  other  cases  of  '14,  -12,  •!,  and  '07  inch.  But 
these  cotyledons,  after  being  extended  horizontally, 
bowed  themselves  upwards  as  effectually  as  the  un- 
niutilated  specimens  in  the  same  pots,  showing  that 
sensitiveness  to  gravitation  is  not  confined  to  their  tips. 

GEOTROPISM. 

This  movement  is  directly  the  reverse  of  apogeo- 
tropism.  Many  organs  bend  downwards  through  epi- 
nasty  or  apheliotropism  or  from  their  own  weight ;  but 
we  have  met  with  very  few  cases  of  a  downward  move- 
ment in  sub-aerial  organs  due  to  geotropism.  We 
shall,  however,  give  one  good  instance  in  the  following 
section,  in  the  case  of  Trifolium  subterraneum,  and 
probably  in  that  of  Arachis  liypogsea. 

On  the  other  hand,  all  roots  which  penetrate  the 
ground  (including  the  modified  root-like  petioles  of 
Megarrhiza  and  Ipomoea  leptophylla)  are  guided  in  their 
downward  course  by  geotropism;  and  so  are  many 
aerial  roots,  whilst  others,  as  those  of  the  Ivy,  appear 
to  be  indifferent  to  its  action.  In  our  first  chapter  the 
movements  of  the  radicles  of  several  seedlings  were 
described.  We  may  there  see  (Fig.  1)  how  a  radicle 
of  the  cabbage,  when  pointing  vertically  upwards  so 
as  to  be  very  little  acted  on  by  geotropism,  circum- 
nutated ;  and  how  another  (Fig.  2)  w?hich  was  at  first 
placed  in  an  inclined  position  bowed  itself  downwards 
in  a  zigzag  line,  sometimes  remaining  stationary  for  a 
time.  Two  other  radicles  of  the  cabbage  travelled 
downwards  in  almost  rectilinear  courses.  A  radicle  of 
the  bean  placed  upright  (Fig.  20)  made  a  great  sweep 
and  zigzagged;  but  as  it  sank  downwards  and  was 
more  strongly  acted  on  by  geotropism,  it  moved  in  an 


CHAP.  X.  GEOTKOPISM.  513 

almost  straight  course.  A  radicle  of  Cucurbita,  directed 
upwards  (Fig.  26),  also  zigzagged  at  first,  and  de- 
scribed small  loops ;  it  then  moved  in  a  straight  line. 
Nearly  the  same  result  was  observed  with  the  radicles 
of  Zea  mays.  But  the  best  evidence  of  the  intimate 
connection  between  circumnutation  and  geotropism 
was  afforded  by  the  radicles  of  Phaseolus,  Vicia,  and 
Quercus,  and  in  a  less  degree  by  those  of  Zea  and 
^sculus  (see  Figs.  18,  19,  21,  41,  and  52) ;  for  when 
these  were  compelled  to  grow  and  slide  down  highly 
inclined  surfaces  of  smoked  glass,  they  left  distinctly 
serpentine  tracks. 

The  Burying  of  Seed-capsules :  Trifolium  subferraneum. — The 
flower-heads  of  this  plant  are  remarkable  from  producing  only 
3  or  4  perfect  flowers,  which  are  situated  exteriorly.  All  the 
other  many  flowers  abort,  and  are  modified  into  rigid  points, 
with  a  bundle  of  vessels  running  up  their  centres.  After  a  time 
5  long,  elastic,  claw-like  projections,  which  represent  the  divi- 
sions of  the  calyx,  are  developed  on  their  summits.  As  soon  as 
the  perfect  flowers  wither  they  bend  downwards,  supposing  the 
peduncle  to  stand  upright,  and  they  then  closely  surround  its 
upper  part.  This  movement  is  due  to  epinasty,  as  is  likewise 
the  case  with  the  flowers  of  T.  repens.  The  imperfect  central 
flowers  ultimately  follow,  one  after  the  other,  the  same  course. 
Whilst  the  perfect  flowers  are  thus  bending  down,  the  whole 
peduncle  curves  downwards  and  increases  much  in  length, 
until  the  flower-head  reaches  the  ground.  Vaucher  *  says  that 
when  the  plant  is  so  placed  that  the  heads  cannot  soon  reach 
the  ground,  the  peduncles  grow  to  the  extraordinary  length  of 
from  6  to  9  inches.  In  whatever  position  the  branches  may  be 
placed,  the  upper  part  of  the  peduncle  at  first  bends  vertically 
upwards  through  heliotropism ;  but  as  soon  as  the  flowers 
begin  to  wither  the  downward  curvature  of  the  whole  peduncle 
commences.  As  this  latter  movement  occurred  in  complete 
darkness,  and  with  peduncles  arising  from  upright  and  from 
dependent  branches,  it  cannot  be  due  to  apheliotropism  or  to 
epinasty,  but  must  be  attributed  to  geotropism.  Nineteen 


*  'Hist.  Phys.  des  Plantes  d'Europe,'  torn,  ii,  1841,  p.  106. 


514  MODIFIED   CIRCUMNUTATION.  CHAP.  X. 

upright  flower-heads,  arising  from  branches  in  all  sorts  of  posi- 
tions, on  plants  growing  in  a  warm  greenhouse,  were  marked 
with  thread,  and  after  24  h.  six  of  them  were  vertically  depen- 
dent ;  these  therefore  had  travelled  through  180°  in  this  time. 
Ten  were  extended  sub-horizontally,  and  these  had  moved 
through  about  90°.  Three  very  young  peduncles  had  as  yet 
moved  only  a  little  downwards,  but  after  an  additional  24.  h. 
were  greatly  inclined. 

At  the  time  when  the  flower-heads  reach  the  ground,  the 
younger  imperfect  flowers  in  the  centre  are  still  pressed  closely 
together,  and  form  a  conical  projection ;  whereas  the  perfect  and 
imperfect  flowers  on  the  outside  are  upturned  and  closely  sur- 
round the  peduncle.  They  are  thus  adapted  to  offer  as  little 
resistance,  as  the  case  admits  of,  in  penetrating  the  ground, 
though  the  diameter  of  the  flower-head  is  still  considerable. 
The  means  by  which  this  penetration  is  effected  will  presently 
be  described.  The  flower-heads  are  able  to  bury  themselves  in 
common  garden  mould,  and  easily  in  sand  or  in  fine  sifted 
cinders  packed  rather  closely.  The  depth  to  which  they  pene- 
trated, measured  from  the  surface  to  the  base  of  the  head,  was 
between  i  and  2  inch,  but  in  one  case  rather  above  0'6  inch. 
With  a  plant  kept  in  the  house,  a  head  partly  buried  itself  in 
sand  in  6  h. :  after  3  days  only  the  tips  of  the  reflexed  calyces 
were  visible,  and  after  6  days  the  whole  had  disappeared.  But 
with  plants  growing  out  of  doors  we  believe,  from  casual  obser- 
vations, that  they  bury  themselves  in  a  much  shorter  time. 

After  the  heads  have  buried  themselves,  the  central  aborted 
flowers  increase  considerably  in  length  and  rigidity,  and 
become  bleached.  They  gradually  curve,  one  after  the  other, 
upwards  or  towards  the  peduncle,  in  the  same  manner  as 
did  the  perfect  flowers  at  first.  In  thus  moving,  the  long  claws 
on  their  summits  carry  with  them  some  earth.  Hence  a  flower- 
head  which  has  been  buried  for  a  sufficient  time,  forms  a  rather 
large  ball,  consisting  of  the  aborted  flowers,  separated  from  one 
another  by  earth,  and  surrounding  the  little  pods  (the  product 
of  the  perfect  flowers)  which  lie  close  round  the  upper  part  of 
the  peduncle.  The  calyces  of  the  perfect  and  imperfect  flowers 
are  clothed  with  simple  and  multicellular  hairs,  which  have  the 
power  of  absorption;  for  when  placed  in  a  weak  solution  of 
carbonate  of  ammonia  (2  gr.  to  1  oz.  of  water)  their  proto- 
plasmic contents  immediately  became  aggregated  and  afterwards 
displayed  the  usual  slow  movements.  This  clover  generally 


CHAP.  X.  GEOTROPISM.  515 

grows  in  dry  soil,  but  whether  the  power  of  absorption  by  the 
hairs  on  the  buried  flower-heads  is  of  any  importance  to  them 
we  do  not  know.  Only  a  few  of  the  flower-heads,  which  from 
their  position  are  not  able  to  reach  the  ground  and  bury  them- 
selves, yield  seeds ;  whereas  the  buried  ones  never  failed,  as  far 
as  we  observed,  to  produce  as  many  seeds  as  there  had  been 
perfect  flowers. 
We  will  now  consider  the  movements  of  the  peduncle  whilst 


Fig.  190. 


TrifoHum  sulterraneum  :  downward  movement  of  peduncle  from  19°  beneath 
the  horizon  to  a  nearly  vertically  dependent  position,  traced  from 
11  A.M.  July  22nd  to  the  morning  of  25th.  Glass  filament  fixed 
transversely  across  peduncle,  at  base  of  flower-head. 

curving  down  to  the  ground.  We  have  seen  in  Chap.  IV., 
Fig.  9'2,  p.  225,  that  an  upright  young  flower-head  circumnu- 
tated  conspicuously;  and  that  this  movement  continued  after 
the  peduncle  had  begun'  to  bend  downwards.  The  same 
peduncle  was  observed  when  inclined  at  an  angle  of  19°  above 
the  horizon,  and  it  circumnutated  during  two  days.  Another 


516 


MODIFIED  CIRCUMNUTATION. 


CHAP.  X. 


Fig.  191 


which  was  already  curved  36°  beneath  the  horizon,  was  observed 
from  11  A.M.  July  22nd  to  the  27th,  by  which  latter  date  it 
had  become  vertically  dependent.  Its  course  during  the  first 
12  h.  is  shown  in  Fig.  190,  and  its  position  on  the  three 
succeeding  mornings  until  the  25th, 
when  it  was  nearly  vertical.  During 
the  first  day  the  peduncle  clearly 
circumnutated,  for  it  moved  4  times 
down  and  3  times  up;  and  on  each 
succeeding  day,  as  it  sank  downwards, 
the  same  movement  continued,  but 
was  only  occasionally  observed  and 
was  less  strongly  marked.  It  should 
^  stated  that  these  peduncles  were 

cumnutating  movement  of  observed  under  a  double  skylight  in 
peduncle,  whilst  the  flower-  the  house,  and  that  they  generally 


TrifoliumMerraneum:  cir- 


Fig.  192. 


of  the  calyx  still  visible  ;   slowly  than  those  on  plants  growing 
traced  from  8  A.M.  July  out  of  doors  or  in  the  greenhouse. 
26th  to   9  A.M    on  27th.       The  movement  of  another  vertically 
v^l"sfiXp«   dependent  peduncle  with  the  flower- 
•near  flower-  head.  head  standing  half  an  inch  above  the 

ground,  was  traced,  and  again  when 

it  first  touched  the   ground;  in   both  cases  irregular  ellipses 
were  described  every  4  or  5  h.     A  peduncle  on  a  plant  which 
had  been  brought  into  the  house, 
moved  from  an  upright  into  a  ver- 
tically   dependent    position    in   a 
single  day;   and    here  the  course 
during  the  first  12  h.  was  nearly 
straight,  but  with  a  few  well-mark*  d 
Trifo!iu,n  subterraneum  :  move-   zigzags  which  betrayed  the  essential 
ment  of  same  peduncle,  with   nature  of  the  movement.    Lastly, 
flower-head  completely  buried   the  circumnutation  of  a  peduncle 

™»  tra-d  *«"*  51h-  whilst  ™ 

the  act  of  burying  itself  obliquely 
in  a  little  heap  of  sand.  After  it  had  buried  itself  to  such  a 
depth  that  the  tips  of  the  sepals  were  alone  visible,  the  above 
figure  (Fig.  191)  was  traced  during  25  h.  When  the  flower- 
head  had  completely  disappeared  beneath  the  sand,  another 
tracing  was  made  during  11  h.  45m.  (Fig.  192);  and  here  again 
we  see  that  the  peduncle  was  circumnutating. 


CHAP.  X.  GEOTEOPISM.  517 

Any  one  who  will  observe  a  flower-head  burying  itself,  will  be 
convinced  that  the  rocking  movement,  due  to  the  continued 
circumnutation  of  the  peduncle,  plays  an  important  part  in  the 
act.  Considering  that  the  flower-heads  are  very  light,  that  the 
peduncles  are  long,  thin,  and  flexible,  and  that  they  arise  from 
flexible  branches,  it  is  incredible  that  an  object  as  blunt  as  one 
of  these  flower-heads  could  penetrate  the  ground  by  means  of 
the  growing  force  of  the  peduncle,  unless  it  were  aided  by  the 
rocking  movement.  After  a  flower-head  has  penetrated  the 
ground  to  a  small  depth,  another  and  efficient  agency  comes  into 
play ;  the  central  rigid  aborted  flowers,  each  terminating  in  five 
long  claws,  curve  up  towards  the  peduncle;  and  in  doing  so 
can  hardly  fail  to  drag  the  head  down  to  a  greater  depth,  aided 
as  this  action  is  by  the  circumnutating  movement,  which  con- 
tinues after  the  flower-head  has  completely  buried  itself.  The 
aborted  flowers  thus  act  something  like  the  hands  of  the  mole, 
which  force  the  earth  backwards  and  the  body  forwards. 

It  is  well  known  that  the  seed-capsules  of  various  widely 
distinct  plants  either  bury  themselves  in  the  ground,  or  are 
produced  from  imperfect  flowers  developed  beneath  the  surface. 
Besides  the  present  case,  two  other  well-marked  instances  will 
be  immediately  given.  It  is  probable  that  one  chief  good  thus 
gained  is  the  protection  of  the  seeds  from  animals  which  prey  on 
them.  In  the  case  of  T.  subterraneum,  the  seeds  are  not  only 
concealed  by  being  buried,  but  are  likewise  protected  by  being 
closely  surrounded  by  the  rigid,  aborted  flowers.  We  may  the 
more  confidently  infer  that  protection  is  here  aimed  at,  because 
the  seeds  of  several  species  in  this  same  genus  are  protected  in 
other  ways  ;*  namely,  by  the  swelling  and  closure  of  the  calyx, 
or  by  the  persistence  and  bending  down  of  the  standard-petal,  &c. 
But  the  most  curious  instance  is  that  of  T.  ylobosum,  in  which 
the  upper  flowers  are  sterile,  as  in  T.  subterraneum,  but  are  here 
developed  into  large  brushes  of  hairs  which  envelop  and  protect 
the  seed-bearing  flowers.  Nevertheless,  in  all  these  cases  the 
capsules,  with  their  seeds,  may  profit,  as  Mr.  T.  Thiselton  Dyer 
has  remarked,!  by  their  being  kept  somewhat  damp ;  and  the 
advantage  of  such  dampness  perhaps  throws  light  on  the  pre- 
sence of  the  absorbent  hairs  on  the  buried  flower-heads  of  T.  sub- 
terraneum.* According  to  Mr.  Bentham,  as  quoted  by  Mr.  Dyer, 


*  Vaucher,   '  Hist.  Phys.    des          t  See  his  interesting  article  in 
Flantps  d'Europe,'  torn.  ii.  p.  110.      '  Nature,'  April  4th,  1878,  p.  446. 


518  MODIFIED  CIRCUMNUTATION.  CHAP.  X. 

the  prostrate  habit  of  Hdiantliemum  prostratum  "  brings  the 
capsules  in  contact  with  the  surface  of  the  ground,  postpones 
their  maturity,  and  so  favours  the  seeds  attaining  a  larger  size." 
The  capsules  of  Cyclamen  and  of  Oxalis  acetostlla  are  only  occa- 
sionally buried,  and  this  only  beneath  dead  leaves  or  moss.  If 
it  be  an  advantage  to  a  plant  that  its  capsules  should  be  kept 
damp  and  cool  by  being  laid  on  the  ground,  we  have  in  these 
latter  cases  the  first  step,  from  which  the  power  of  penetrating 
the  ground,  with  the  aid  of  the  always  present  movement  of 
circumnutation,  might  afterwards  have  been  gained. 

Arachis  hypogcea. — The  flowers  which  bury  themselves,  rise 
from  stiff  branches  a  few  inches  above  the  ground,  and  stand 
upright.  After  they  have  fallen  off,  the  gynophore,  that  is  the 
part  which  supports  the  ovarium,  grows  to  a  great  length,  even 
to  3  or  4  inches,  and  bends  perpendicularly  downwards.  It 
resembles  closely  a  peduncle,  but  has  a  smooth  and  pointed 
apex,  which  contains  the  ovules,  and  is  at  first  not  in  the  least 
enlarged.  The  apex  after  reaching  the  ground  penetrates  it,  in 
one  case  observed  by  us  to  a  depth  of  1  inch,  and  in  another 
to  0'7  inch.  It  there  becomes  developed  into  a  large  pod. 
Flowers  which  are  seated  too  high  on  the  plant  for  the  gyno- 
phore to  reach  the  ground  are  said  *  never  to  produce  pods. 

The  movement  of  a  young  gynophore,  rather  under  an  inch 
in  length  and  vertically  dependent,  was  traced  during  46  h.  by 
means  of  a  glass  filament  (with  sights)  fixed  transversely  a 
little  above  the  apex.  It  plainly  circumnutated  (Fig.  193) 
whilst  increasing  in  length  and  growing  downwards.  It  was 
then  raised  up,  so  as  to  be  extended  almost  horizontally,  and 
the  terminal  part  curved  itself  downwards,  following  a  nearly 
straight  course  during  12  h.,  but  with  one  attempt  to  circum- 
nutate,  as  shown  in  Fig.  194.  After  24  h.  it  had  become  nearly 
vertical.  Whether  the  exciting  cause  of  the  downward  move- 
ment is  geotropism  or  apheliotropism  was  not  ascertained ;  but 
probably  it  is  not  apheliotropism,  as  all  the  gynophores  grew 
straight  down  towards  the  ground,  whilst  the  light  in  the  hot- 
house entered  from  one  side  as  well  as  from  above.  Another 
and  older  gynophore,  the  apex  of  which  had  nearly  reached  the 
ground,  was  observed  during  3  days  in  the  same  manner  as  the 
first-mentioned  short  one;  and  it  was  found  to  be  always  circum- 
nutating.  During  the  first  34  h.  it  described  a  figure  which 

*  « Gard.  Chronicle,'  1857,  p.  5C6. 


CHAP.  X. 


GEOTROPISM. 


519 


represented  four  ellipses.     Lastly,  a  long  gynophore,  the  apex  of 
which  had  buried  itself  to  the  depth  of  about  half  an  inch,  was 

Fig.  194. 


Fig.  193. 


Arachis  hypogcei :  circiim- 
nutation  of  vertically 
dependent  young  gyno- 
phore, traced  on  a  ver- 
tical glass  from  10  A.M. 
July  31st  to  8  A.M.  Aug. 
2nd. 


Arachis  hypogcea :  down- 
ward movement  of  same 
young  gynophore,  after 
being  extended  horizon- 
tally;  traced  on  a  vertical 
glass  from  8.30  A.M.  to 
8.30  P.M.  Aug.  2nd. 


pulled  up  and  extended  horizontally :  it  quickly  began  to  curve 
downwards  in  a  zigzag  line ;  but  on  the  following  day  the  tor- 


520  MODIFIED  CIRCUMNUTATION.  CHAP.  X. 

minal  bleached  portion  was  a  little  shrivelled.  As  the  gyno- 
phores  are  rigid  and  arise  from  stiff  branches,  and  as  they 
terminate  in  sharp  smooth  points,  it  is  probable  that  they  could 
penetrate  the  ground  by  the  mere  force  of  growth.  But  this 
action  must  be  aided  by  the  circumnutating  movement,  for  fine 
sand,  kept  moist,  was  pressed  close  round  the  apex  of  a  gyno- 
phore  which  had  reached  the  ground,  and  after  a  few  hours  it 
was  surrounded  by  a  narrow  open  crack.  After  three  weeks 
this  gynophore  was  uncovered,  and  the  apex  was  found  at  a 
depth  of  rather  above  half  an  inch  developed  into  a  small,  white, 
oval  pod. 

Amphicarpcea  monoica. — This  plant  produces  long  thin  shoots, 
which  twine  round  a  support  and  of  course  circumnutate. 
Early  in  the  summer  shorter  shoots  are  produced  from  the 
lower  parts  of  the  plant,  which  grow  perpendicularly  downwards 
and  penetrate  the  ground.  One  of  these,  terminating  in  a 
minute  bud,  was  observed  to  bury  itself  in  sand  to  a  depth  of 
0'2  inch  in  24  h.  It  was  lifted  up  and  fixed  in  an  inclined 
position  about  25°  beneath  the  horizon,  being  feebly  illuminated 
from  above.  In  this  position  it  described  two  vertical  ellipses 
in  24  h. ;  but  on  the  following  day,  when  brought  into  the  house, 
it  circumnutated  only  a  very  little  round  the  same  spot.  Other 
branches  were  seen  to  penetrate  the  ground,  and  were  after- 
wards found  running  like  roots  beneath  the  surface  for  a  length 
of  nearly  two  inches,  and  they  had  grown  thick.  One  of  these, 
.after  thus  running,  had  emerged  into  the  air.  How  far  circum- 
nutation  aids  these  delicate  branches  in  entering  the  ground  we 
do  not  know ;  but  the  reflexed  hairs  with  which  they  are  clothed 
will  assist  in  the  work.  This  plant  produces  pods  in  the  air, 
and  others  beneath  the  ground ;  which  differ  greatly  in  appear- 
ance. Asa  Gray  says  *  that  it  is  the  imperfect  flowers  on  the 
creeping  branches  near  the  base  of  the  plant  which  produce  the 
subterranean  pods;  these  flowers,  therefore,  must  bury  them- 
selves like  those  of  Arachis.  But  it  may  be  suspected  that  the 
branches  which  were  seen  by  us  to  penetrate  the  ground  also 
produce  subterranean  flowers  and  pods. 

DlAGEOTKOriSM. 

Besides  geotropism  and  apogeotropism,  there  is, 
according  to  Frank,  an  allied  form  of  movement, 

*  « Manual  of  the  Botany  of  the  Northern  United  States,'  1856,  p.  106. 


CHAP.  X.  DIAGEOTKOPISM.  521 

namely,  "  transverse-geotropism,"  or  diageotropism,  as 
we  may  call  it  for  the  sake  of  matching  our  other 
terms.  Under  the  influence  of  gravitation  certain 
parts  are  excited  to  place  themselves  more  or  less 
transversely  to  the  line  of  its  action.*  We  made  no 
observations  on  this  subject,  and  will  here  only  re- 
mark that  the  position  of  the  secondary  radicles  of 
various  plants,  which  extend  horizontally  or  are  a 
little  inclined  downwards,  would  probably  be  con- 
sidered by  Frank  as  due  to  transverse-geotropism. 
As  it  has  been  shown  in  Chap.  I.  that  the  secondary 
radicles  of  Cucurbita  made  serpentine  tracks  on  a 
smoked  glass-plate,  they  clearly  circumnutated, 
and  there  can  hardly  be  a  doubt  that  this  holds 
good  with  other  secondary  radicles.  It  seems  there- 
fore highly  probable  that  they  place  themselves  in 
their  diageotropic  position  by  means  of  modified 
circurnnutation. 

Finally,  we  may  conclude  that  the  three  kinds  of 
movement  which  have  now  been  described  and  which 
are  excited  by  gravitation,  consist  of  modified  circurn- 
nutation. Different  parts  or  organs  on  the  same  plant, 
and  the  same  part  in  different  species,  are  thus  excited 
to  act  in  a  widely  different  manner.  We  can  see  no 
reason  why  the  attraction  of  gravity  should  directly 
modify  the  state  of  turgescence  and  subsequent  growth 
of  one  part  on  the  upper  side  and  of  another  part  on 
the  lower  side.  We  are  therefore  led  to  infer  that  both 
geotropic,  apogeotropic,  and  diageotropic  movements, 
the  purpose  of  which  we  can  generally  understand, 


*  Elfving  has  lately  described  excellent  instance  of  such  move- 
''Arbtiten  des  Bot.  instituts  in  menls  in  the  ihizomes  of  certain 
Wurzmirg.'  13.  ii.  1880,  p.  4s9)  an  plants. 


522  MODIFIED   CIHCUMNUTATION.  CHAP  X. 

have  been  acquired  for  the  advantage  of  the  plant  by 
the  modification  of  the  ever-present  movement  of 
circumnutation.  This,  however,  implies  that  gravi- 
tation produces  some  effect  on  the  young  tissues 
sufficient  to  serve  as  a  guide  to  the  plant. 


CHAP.  XL      SENSITIVENESS   TO   GRAVITATION.  523 


CHAPTEK  XI. 

LOCALISED  SENSITIVENESS  TO  GRAVITATION,  AND  ITS  TRANSMITTED 
EFFECTS. 

General  considerations — Vicia  fuba,  effects  of  amputating  the  tips  of 
the  radicles — Regeneration  of  the  tips — Effects  of  a  short  exposure 
of  the  tips  to  geotropic  action  and  their  subsequent  amputation — 
Effects  of  amputating  the  tips  obliquely — Effects  of  cauterising  the 
tips — Effects  of  grease  on  the  tips — Pisum  sativum,  tips  of  radicles 
cauterised  transversely,  and  on  their  upper  and  lower  sides — 
Phaseolus,  cauterisation  and  grease  on  the  tips — Gossypium — 
Cucurbita,  tips  cauterised  transversely,  and  on  their  upper  and 
lower  sides  —  Zea,  tips  cauterised  —  Concluding  remarks  and 
summary  of  chapter — Advantages  of  the  sensibility  to  geotropism 
being  localised  in  the  tips  of  the  radicles. 

CIESIELSKI  states  *  that  when  the  roots  of  Pisum, 
Lens  and  Yicia  were  extended  horizontally  with  their 
tips  cut ,  off,  they  were  not  acted  on  by  geotropism ; 
but  some  days  afterwards,  when  a  new  root-cap  and 
vegetative  point  had  been  formed,  they  bent  them- 
selves perpendicularly  downwards.  He  further  states 
that  if  the  tips  are  cut  off,  after  the  roots  have  been 
left  extended  horizontally  for  some  little  time,  but 
before  they  have  begun  to  bend  downwards,  they  may 
be  placed  in  any  position,  and  yet  will  bend  as  if  still 
acted  on  by  geotropism ;  and  this  shows  that  some 
influence  had  been  already  transmitted  to  the  bending 
part  from  the  tip  before  it  was  amputated.  Sachs 
repeated  these  experiments ;  he  cut  off  a  length  of 
between  *05  and  1  mm.  (measured  from  the  apex  of  the 


*  '  Abwartskriimmung  der  Wurzel,'  luaug.  Dissert.  Breslau.  1871, 
p.  20. 


524  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XI. 

vegetative  point)  of  the  tips  of  the  radicles  of  the 
bean  (Vicia  fdba),  and  placed  them  horizontally  or 
vertically  in  damp  air,  earth,  and  water,  with  the 
result  that  they  became  bowed  in  all  sorts  of  direc- 
tions.* He  therefore  disbelieved  in  Ciesielski's  con- 
clusions. But  as  we  have  seen  with  several  plants 
that  the  tip  of  the  radicle  is  sensitive  to  contact  and 
to  other  irritants,  and  that  it  transmits  some  influence 
to  the  upper  growing  part  causing  it  to  bend,  there 
seemed  to  us  to  be  no  a  priori  improbability  in 
Ciesielski's  statements.  We  therefore  determined  to 
repeat  his  experiments,  and  to  try  others  on  several 
species  by  different  methods. 

Vicia  f aba. — Eadicles  of  this  plant  were  extended  horizontally 
either  over  water  or  with  their  lower  surfaces  just  touching  it. 
Their  tips  had  previously  been  cut  oil",  in  a  direction  as  accu- 
rately transverse  as  could  be  done,  to  different  lengths,  measured 
from  the  apex  of  the  root-cap,  and  which  will  be  specified  in 
each  case.  Light  was  always  excluded.  We  had  previously 
tried  hundreds  of  unmutilated  radicles  under  similar  circum- 
stances, and  found  that  every  one  that  was  healthy  became 
plainly  geotropic  in  under  12  h.  In  the  case  of  four  radicles 
which  had  their  tips  cut  off  for  a  length  of  1*5  mm.,  new  root- 
caps  and  new  vegetative  points  were  re-formed  after  an  interval 
of  3  days  20  h. ;  and  these  when  placed  horizontally  were  acted 
on  by  geotropism.  On  some  other  occasions  this  regeneration 
of  the  tips  and  reacquired  sensitiveness  occurred  within  a  some- 
what shorter  time.  Therefore,  radicles  having  their  tips 
amputated  should  be  observed  in  from  12  to  48  h.  after  the 
operation. 

Four  radicles  were  extended  horizontally  with  their  lower 
surfaces  touching  the  water,  and  with  their  tips  cut  off  for  a 
length  of  only  0'5  mm. :  after  23  h.  three  of  them  were  still 
horizontal ;  after  47  h.  one  of  the  three  became  fairly  geotropic ; 
and  after  70  h.  the  other  two  showed  a  trace  of  this  action.  The 
fourth  radicle  was  vertically  geotropic  after  23  h. ;  but  by  an 


'  Arbeiten  des  Bot.  Institute  in  AViirzburg,'  Heft.  iii.  1873,  p.  432. 


CHAP.  XL        TKANSMITTED   EFFECTS  :   VICIA.  525 

accident  the*  root-cap  alone  and  not  the  vegetative  point  was 
found  to  have  been  amputated  ;  so  that  this  case  formed  no  real 
exception  arid  might  have  been  excluded. 

Five  radicles  were  extended  horizontally  like  the  last,  and 
had  their  tips  cut  off  for  a  length  of  1  mm. ;  after  22-23  h.,  four 
of  them  were  still  horizontal,  and  one  was  slightly  geotropic ; 
after  48  h.  the  latter  had  become  vertical ;  a  second  was  also 
somewhat  geotropic ;  two  remained  approximately  horizontal ; 
and  the  last  or  fifth  had  grown  in  a  disordered  manner,  for  it 
was  inclined  upwards  at  an  angle  of  65°  above  the  horizon. 

Fourteen  radicles  were  extended  horizon  tally  at  a  little  height 
over  the  water  with  their  tips  cut  off  for  a  length  of  T5  mm. ; 
after  12  h.  all  were  horizontal,  whilst  five  control  or  standard 
specimens  in  the  same  jar  were  all  bent  greatly  downwards. 
After  24  h.  several  of  the  amputated  radicles  remained  hori- 
zontal, but  some  showed  a  trace  of  geotropism,  and  one  was 
plainly  geotropic,  for  it  was  inclined  at  40°  beneath  the  horizon. 

Seven  horizontally  extended  radicles  from  which  the  tips  had 
been  cut  off  for  the  unusual  length  of  2  mm.  unfortunately  were 
not  looked  at  until  35  h.  had  elapsed ;  three  w^ere  still  horizontal, 
but,  to  our  surprise,  four  were  more  or  less  plainly  geotropic. 

The  radicles  in  the  foregoing  cases  were  measured  before  their 
tips  were  amputated,  and  in  the  course  of  24  h.  they  had  all 
increased  greatly  in  length;  but  the  measurements  are  not 
worth  giving.  It  is  of  more  importance  that  Sachs  found  that 
the  rate  of  growth  of  the  different  parts  of  radicles  with 
amputated  tips  was  the  same  as  with  unmutilated  ones.  Alto  - 
gether  twenty-nine  radicles  were  operated  on  in  the  manner 
above  described,  and  of  these  only  a  few  showed  any  geotropic 
curvature  within  24  h. ;  whereas  radicles  with  unmutilated  tips 
always  became,  as  already  stated,  much  bent  down  in  less  than 
half  of  this  time.  The  part  of  the  radicle  which  bends  most  lies 
at  the  distance  of  from  3  to  6  mm.  from  the  tip,  and  as  the 
bending  part  continues  to  grow  after  the  operation,  there  does 
not  seem  any  reason  why  it  should  not  have  been  acted  on  by 
geotropism,  unless  its  curvature  depended  on  some  influence 
transmitted  from  the  tip.  And  we  have  clear  evidence  of  such 
transmission  in  Ciesielski's  experiments,  which  we  repeated  and 
extended  in  the  following  manner. 

Beans  were  embedded  in  friable  peat  with  the  hilum  down- 
wards, and  after  their  radicles  had  grown  perpendicularly  down 
for  a  length  of  from  ^  to  1  inch,  sixteen  were  selected  which 


526  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XI. 

were  perfectly  straight,  and  these  were  placed  horizontally  on 
the  peat,  being  covered  by  a  thin  layer  of  it.  They  were  thus 
left  for  an  average  period  of  1  h.  37  m.  The  tips  were  then  cut 
off  transversely  for  a  length  of  1*5  mm.,  and  immediately  after- 
wards they  were  embedded  vertically  in  the  peat.  In  this  position 
geotropism  would  not  tend  to  induce  any  curvature,  but  if  some 
influence  had  already  been  transmitted  from  the  tip  to  the  part 
which  bends  most,  we  might  expect  that  this  part  would  become 
curved  in  the  direction  in  which  geotropism  had  previously 
acted;  for  it  should  be  noted  that  these  radicles  being  now 
destitute  of  their  sensitive  tips,  would  not  be  prevented  by 
geotropism  from  curving  in  any  direction.  The  result  was  that 
of  the  sixteen  vertically  embedded  radicles,  four  continued  for 
several  days  to  grow  straight  downwards,  whilst  twelve  became 
more  or  less  bowed  laterally.  In  two  of  the  twelve,  a  trace  of 
curvature  was  perceptible  in  3  h.  30  m.,  counting  from  the  time 
when  they  had  first  been  laid  horizontally  ;  and  all  twelve  were 
plainly  bowed  in  6  h.,  and  still  more  plainly  in  9  h.  In  every 
one  of  them  the  curvature  was  directed  towards  the  side  which 
had  been  downwards  whilst  the  radicles  remained  horizontal. 
The  curvature  extended  for  a  length  of  from  5  to,  in  one  in- 
stance, 8  mm.,  measured  from  the  cut-off  end.  Of  the  twelve 
bowed  radicles  five  became  permanently  bent  into  a  right  angle ; 
the  other  seven  were  at  first  much  less  bent,  and  their  ciirvature 
generally  decreased  after  24  h.,  but  did  not  wholly  disappear. 
This  decrease  of  curvature  would  naturally  follow,  if  an  ex- 
posure of  only  1  h.  37  m.  to  geotropism,  served  to  modify  the 
turgescence  of  the  cells,  but  not  their  subsequent  growth  to 
the  full  extent.  The  five  radicles  which  were  rectangularly 
bent  became  fixed  in  this  position,  and  they  continued  to  grow- 
out  horizontally  in  the  peat  for  a  length  of  about  1  inch  during 
from  4  to  6  days.  By  this  time  new  tips  had  been  formed ;  and 
it  should  be  remarked  that  this  regeneration  occurred  slower  in 
the  peat  than  in  water,  owing  perhaps  to  the  radicles  being 
often  looked  at  and  thus  disturbed.  After  the  tips  had  been 
regenerated,  geotropism  was  able  to  act  on  them,  so  that  they 
now  became  bowed  vertically  downwards.  An  accurate  draw- 
ing (Fig.  195)  is  given  on  the  opposite  page  of  one  of  these  five 
radicles,  reduced  to  half  the  natural  size. 

We  next  tried  whether  a  shorter  exposure  to  geotropism 
would  sufiice  to  produce  an  after-effect.  Seven  radicles  were 
extended  horizontally  for  an  hour,  instead  of  1  h.  37  m.  as  in  the 


CHAP.  XI.       TRANSMITTED  EFFECTS  :   VICIA. 


527 


Fig.  195. 


former  trial ;  and  after  their  tips  (1*5  mm.  in  length)  had  been 
amputated,  they  were  placed  vertically  in  damp  peat.  Of  these, 
three  were  not  in  the  least  affected  and  continued  for  days  to 
grow  straight  downwards.  Four  showed  after  8  h.  30  m.  a  mere 
trace  of  curvature  in  the  direction  in  which  they  had  been  acted 
on  by  geotropism ;  and  in  this  respect  they  differed  much  from 
those  which  had  been  exposed  for 
1  h.  37  m.,  for  many  of  the  latter 
were  plainly  curved  in  6  h.  THe 
curvature  of  one  of  these  four 
radicles  almost  disappeared  after 
24  h.  In  the  second,  the  cur- 
vature increased  during  two  days 
and  then  decreased.  The  third 
radicle  became  permanently  bent, 
so  that  its  terminal  part  made  an 
angle  of  about  45°  with  its  original 
vertical  direction.  The  fourth 
radicle  became  horizontal.  These 
two  latter  radicles  continued 
during  two  more  days  to  grow 
in  the  peat  in  the  same  directions, 
that  is,  at  an  angle  of  45°  be- 
neath the  horizon  and  horizon- 
tally. By  the  fourth  morning  new 
tips  had  been  re-formed,  and  now 
geotropism  was  able  to  act  on 
them  again,  and  they  became 
bent  perpendicularly  downwards, 
exactly  as  in  the  case  of  the 
five  radicles  described  in  the 
last  paragraph  and  as  is  shown  in 
the  figure  (Fig.  195)  here  given. 

Lastly,  five  other  radicles  were  similarly  treated,  but  were  ex- 
posed to  geotropism  during  only  45  m.  After  8  h.  30  m.  only 
one  was  doubtfully  affected;  after  24  h.  two  were  just  per- 
ceptibly curved  towards  the  side  which  had  been  acted  on  by 
geotropism ;  after  48  h.  the  one  first  mentioned  had  a  radius  of 
curvature  of  60  mm.  That  this  curvature  was  due  to  the  action 
of  geotropism  during  the  horizontal  position  of  the  radicle,  was 
shown  after  4  days,  when  a  new  tip  had  been  reformed,  for  it 
then  grew  perpendicularly  downwards.  We  learn  from  this 


Vicia  faba  :  radicle,  rectangularly 
bent  at  A,  after  the  amputation 
of  the  tip,  due  to  the  previous 
influence  of  geotropism.  L,  side 
of  bean  which  lay  on  the  peat, 
whilst  geotropism  acted  on  the 
radicle.  A,  point  of  chief  cur- 
vature of  the  radicle,  whilst 
standing  vertically  downwards. 
B,  point  of  chief  curvature  after 
the  regeneration  of  the  tip,  when 
geotropism  again  acted.  C,  re- 
generated tip. 


528  SENSITIVENESS   TO   GRAVITATION.        CHAP.  XL 

case  that  when  the  tips  are  amputated  after  an  exposure  to  geo- 
tropism  of  only  45  m.,  though  a  slight  influence  is  sometimes 
transmitted  to  the  adjoining  part  of  the  radicle,  yet  this  seldom 
suffices,  and  then  only  slowly,  to  induce  even  moderately  well- 
pronounced  curvature. 

In  the  previously-  given  experiments  on  29  horizontally  ex- 
tended radicles  with  their  tips  amputated,  only  one  grew  irre- 
gularly in  any  marked  manner,  and  this  became  bowed  upwards 
at  an  angle  of  65°.  In  Ciesielski's  experiments  the  radicles 
could  not  have  grown  very  irregularly,  for  if  they  had  done 
so,  he  could  not  have  spoken  confidently  of  the  obliteration 
of  all  geotropic  action.  It  is  therefore  remarkable  that  Sachs, 
who  experimented  on  many  radicles  with  their  tips  amputated, 
found  extremely  disordered  growth  to  be  the  usual  result.  As 
horizontally  extended  radicles  with  amputated  tips  are  some- 
times acted  on  slightly  by  geotropism  within  a  short  time,  and 
are  often  acted  on  plainly  after  one  or  two  days,  we  thought 
that  this  influence  might  possibly  prevent  disordered  growth, 
though  it  was  not  able  to  induce  immediate  curvature.  There- 
fore 13  radicles,  of  which  6  had  their  tips  amputated  trans- 
versely for  a  length  of  T5  mm.,  and  the  other  7  for  a  length  of 
only  0'5  mm.,  were  suspended  vertically  in  damp  air,  in  which 
position  they  would  not  be  affected  by  geotropism;  but  they 
exhibited  no  great  irregularity  of  growth,  whilst  observed 
during  4  to  6  days.  We  next  thought  that  if  care  were  not 
taken  in  cutting  off  the  tips  transversely,  one  side  of  the  stump 
might  be  irritated  more  than  the  other,  either  at  first  or  sub- 
sequently during  the  regeneration  of  the  tip,  and  that  this 
might  cause  the  radicle  to  bend  to  one  side.  It  has  also  been 
shown  in  Chapter  III.  that  if  a  thin  slice  be  cut  off  one  side 
of  the  tip  of  the  radicle,  this  causes  the  radicle  to  bend  from 
the  sliced  side.  Accordingly,  30  radicles,  with  tips  amputated 
for  a  length  of  1-5  mm.,  were  allowed  to  grow  perpendicularly 
downwards  into  water.  Twenty  of  them  were  amputated  at  an 
angle  of  20°  with  a  line  transverse  to  their  longitudinal  axes ; 
and  such  stumps  appeared  only  moderately  oblique.  The 
remaining  ten  radicles  were  amputated  at  an  angle  of  about 
45°.  Under  these  circumstances  no  less  than  19  out  of  the  30 
became  much  distorted  in  the  course  of  2  or  3  days.  Eleven 
other  radicles  were  similarly  treated,  excepting  that  only  1  mm. 
(including  in  this  and  all  other  cases  the  root-cap)  was  ampu- 
tated ;  and  of  these  only  one  grew  much  and  two  others  slightly 


CHAP.  XL       TRANSMITTED   EFFECTS:   VICIA.  529 

distorted ;  so  that  this  amount  of  oblique  amputation  was  not; 
sufficient.  Out  of  the  above  30  radicles,  only  one  or  two  showed 
in  the  first  24  h.  any  distortion,  but  this  became  plain  in  the 
19  cases  on  the  second  day,  and  still  more  conspicuous  at  the 
close  of  the  third  day,  by  which  time  new  tips  had  been  partially 
or  completely  regenerated.  When  therefore  a  new  tip  is  re- 
formed on  an  oblique  stump,  it  probably  is  developed  sooner  on 
one  side  than  on  the  other :  and  this  in  some  manner  excites 
the  adjoining  part  to  bend  to  one  side.  Hence  it  seems  probable 
that  Sachs  unintentionally  amputated  the  radicles  on  which  he 
experimented,  not  strictly  in  a  transverse  direction. 

This  explanation  of  the  occasional  irregular  growth  of  radicles 
with  amputated  tips,  is  supported  by  the  results  of  cauterising 
their  tips;  for  often  a  greater  length  on  one  side  than  on  the 
other  was  unavoidably  injured  or  killed.  It  should  be  re- 
marked that  in  the  following  trials  the  tips  were  first  dried 
with  blotting-paper,  and  then  slightly  rubbed  with  a  dry  stick 
of  nitrate  of  silver  or  lunar  caustic.  A  few  touches  with  the 
caustic  suffice  to  kill  the  root-cap  and  some  of  the  upper  layers 
of  cells  of  the  vegetative  point.  Twenty-seven  radicles,  some 
young'and  very  short,  others  of  moderate  length,  were  suspended 
vertically  over  water,  after  being  thus  cauterised.  Of  these  some 
entered  the  water  immediately,  and  others  on  the  second  day. 
The  same  number  of  uncauterised  radicles  of  the  same  age 
were  observed  as  controls.  After  an  interval  of  three  or  four 
days  the  contrast  in  appearance  between  the  cauterised  and 
control  specimens  was  wonderfully  great.  The  controls  had 
grown  straight  downwards,  with  the  exception  of  the  normal 
curvature,  which  we  have  called  Sachs'  curvature.  Of  the 
27  cauterised  radicles,  15  had  become  extremely  distorted ;  6  of 
them  grew  upwards  and  formed  hoops,  so  that  their  tips  some- 
times came  into  contact  with  the  bean  above;  5  grew  out 
rectangularly  to  one  side  ;  only  a  few  of  the  remaining  12  were 
quite  straight,  and  some  of  these  towards  the  close  of  our 
observations  became  hooked  at  their  extreme  lower  ends. 
Radicles,  extended  horizontally  instead  of  vertically,  with  their 
tips  cauterised,  also  sometimes  grew  distorted,  but  not  so  com- 
monly, as  far  as  we  could  judge,  as  those  suspended  vertically ; 
for  this  occurred  with  only  5  out  of  19  radicles  thus  treated. 

Instead  of  cutting  off  the  tips,  as  in  the  first  set  of  experi- 
ments, we  next  tried 'the  effects  of  touching  horizontally  ex- 
tended radicles  with  caustic  in  the  manner  just  described.  But 


530  SENSITIVENESS   TO  GEAVITAT1ON.       CHAP  XL 

some  preliminary  remarks  must  first  be  made.  It  may  be  ob- 
jected that  the  caustic  would  injure  the  radicles  and  prevent  them 
from  bending ;  but  ample  evidence  was  given  in  Chapter  III. 
that  touching  the  tips  of  vertically  suspended  radicles  with 
caustic  on  one  side,  does  not  stop  their  bending;  on  the 
contrary,  it  causes  them  to  bend  from  the  touched  side.  We 
also  tried  touching  both  the  upper  and  the  lower  sides  of  the 
tips  of  some  radicles  of  the  bean,  extended  horizontally  in  damp 
friable  earth.  The  tips  of  three  were  touched  with  caustic  on 
their  upper  sides,  and  this  would  aid  their  geotropic  bending ; 
the  tips  of  three  were  touched  on  their  lower  sides,  which 
would  tend  to  counteract  the  bending  downwards ;  and  three 
were  left  as  controls.  After  24  h.  an  independent  observer  was 
asked  to  pick  out  of  the  nine  radicles,  the  two  which  were  most 
and  the  two  which  were  least  bent ;  he  selected  as  the  latter, 
two  of  those  which  had  been  touched  on  their  lower  sides,  and 
as  the  most  bent,  two  of  those  which  had  been  touched  on  the 
upper  side.  Hereafter  analogous  and  more  striking  experiments 
with  Pisum  tativum  and  Cucurbita  ovifera  will  be  given.  We 
may  therefore  safely  conclude  that  the  mere  application  of 
caustic  to  the  tip  does  not  prevent  the  radicles  from  bending. 

In  the  following  experiments,  tne  tips  of  young  horizontally 
extended  radicles  were  just  touched  with  a  stick  of  dry  caustic ; 
and  this  was  held  transversely,  so  that  the  tip  might  be  cau- 
terised all  round  as  symmetrically  as  possible.  The  radicles 
were  then  suspended  in  a  closed  vessel  over  water,  kept  rather 
cool,  viz.,  55°-59°  F.  This  was  done  because  we  had  found 
that  the  tips  were  more  sensitive  to  contact  under  a  low  than 
under  a  high  temperature ;  and  we  thought  that  the  same  rule 
might  apply  to  geotropism.  In  one  exceptional  trial,  nine 
radicles  (which  were  rather  too  old,  for  faey  had  grown  to  a 
length  of  from  3  to  5  cm.),  were  extended  horizontally  in  damp 
friable  earth,  after  their  tips  had  been  cauterised,  and  were 
kept  at  too  high  a  temperature,  viz.,  of  08"  F.,  or  20°  C.  The 
result  in  consequence  was  not  so  striking  as  in  the  subsequent 
cases;  for  although  when  after  9  h.  40  m.  six  of  them  were 
examined,  these  did  not  exhibit  any  geotropic  bending,  yet  after 
24  h.,  when  all  nine  were  examined,  only  two  remained  hori- 
zontal, two  exhibited  a  trace  of  geotropism,  and  five  were 
slightly  or  moderately  geotropic,  yet  not  comparable  in  degree 
with  the  control  specimens.  Marks  had  been  made  on  seven  of 
these  cauterised  radicles  at  10  mm.  from  the  tips,  which  includes 


CiiAr.  XI.        TRANSMITTED   EFFECTS  :   VICIA. 


531 


the  whole  growing  portion ;  and  after  the  24  h.  this  part  had 
a  mean  length  of  37  mm.,  so  that  it  had  increased  to  more 
than  ST  times  its  original  length;  but  it  should  be  remembered 
that,  these  beans  had  been  exposed  to  a  rather  high  temperature. 
Nineteen  young  radicles  with  cauterised  tips  were  extended 
at  different  times  horizontally  over  water.  In  every  trial  an 
equal  number  of  control  specimens  were  observed.  In  the  first 
trial,  the  tips  of  three  radicles  were  lightly  touched  with  the 
caustic  for  6  or  7  seconds,  which  was  a  longer  application  than 
usual.  After  23  h.  30  m.  (temp.  55°-56°  F.)  these  three  radicles, 


A. 


Vicia  faba .  state  of  radicles  which  had  been  extended  horizontally  for 
23  h.  30  m. :  A,  B,  C,  tips  touched  with  caustic  ;  D,  E,  F,  tips  uncaute- 
rised.  Lengths  of  radicles  reduced  to  one-half  scale,  but  by  an  accident 
the  beans  themselves  not  reduced  in  the  same  degree. 

A,  B,  C  (Fig.  196),  were  still  horizontal,  whilst  the  three  control 
specimens  had  become  within  8  h.  slightly  geotropic,  and 
strongly  so  (D,  E,  F)  in  23  h.  30  m.  A  dot  had  been  made  on 
all  six  radicles  at  10  mm.  from  their  tips,  when  first  placed 
horizontally.  After  the  23  h.  30  m.  this  terminal  part,  originally 
10  mm.  in  length,  had  increased  in  the  cauterised  specimens  to 
a-  mean  length  of  17*3  mm.,  and  to  15 '7  mm.  in  the  control 
radicles,  as  shown  in  the  figures  by  the  unbroken  transverse 
line ;  the  dotted  line  being  at  10  mm.  from  the  apex.  The  con- 
trol or  uncauterised  radicles,  therefore,  had  actually  grown  less 


532  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XL 

than  the  cauterised;  but  this  no  doubt  was  accidental,  for 
radicles  of  different  ages  grow  at  different  rates,  and  the  growth 
of  different  individuals  is  likewise  affected  by  unknown  causes. 
The  state  of  the  tips  of  these  three  radicles,  which  had  been 
cauterised  for  a  rather  longer  time  than  usual,  was  as  follows  : 
the  blackened  apex,  or  the  part  which  had  been  actually  touched 
by  the  caustic,  was  succeeded  by  a  yellowish  zone,  due  probably 
to  the  absorption  of  some  of  the  caustic;  in  A,  both  zones 
together  were  1*1  mm.  in  length,  and  1'4  mm.  in  diameter  at  the 
base  of  the  yellowish  zone ;  in  B,  the  length  of  both  was  only 
0 '  7  mm.,  and  the  diameter  0 '  7  mm. ;  in  C,  the  length  was  0  •  8 
mm.,  and  the  diameter  1  '2  mm. 

Three  other  radicles,  the  tips  of  which  had  been  touched  with 
caustic  during  2  or  3  seconds,  remained  (temp.  58°-59°  F.) 
horizontal  for  23  h. ;  the  control  radicles  having,  of  course, 
become  geotropic  within  this  time.  The  terminal  growing  part, 
10  mm.  in  length,  of  the  cauterised  radicles  had  increased  in 
this  interval  to  a  mean  length  of  24 '5  mm.,  and  of  the  controls 
to  a  mean  of  26  mm.  A  section  of  one  of  the  cauterised  tips 
showed  that  the  blackened  part  was  0  •  5  mm.  in  length,  of  which 
0'2mm.  extended  into  the  vegetative  point;  and  a  faint  dis- 
coloration could  be  detected  even  to  1  •  6  mm.  from  the  apex  of 
the  root-cap. 

In  another  lot  of  six  radicles  (temp.  55°-57°  F.)  the  three 
control  specimens  were  plainly  geotropic  in  82  h. ;  and  after  24  h. 
the  mean  length  of  their  terminal  part  had  increased  from 
10  mm.  to  21  mm.  When  the  caustic  was  applied  to  the  three 
cauterised  specimens,  it  was  held  quite  motionless  during 

5  seconds,  and  the  result  was  that  the  black  marks  were  ex- 
tremely minute.     Therefore,  caustic  was  again  applied,  after 
82  h.,  during  which  time  no  geotropic  action  had  occurred. 
When  the   specimens   were    re-examined  after  an   additional 
interval  of  15 £  h.,  one  was  horizontal  and  the  other  two  showed, 
to  our  surprise,  a  trace  of  geotropism  which  in  one  of  them 
soon  afterwards  became  strongly  marked;   but  in  this  latter 
specimen  the  discoloured  tip  was  only  f  mm.  in  length.     The 
growing  part  of  these  three  radicles  increased  in  24  h.  from 
10  mm.  to  an  average  of  16 '5  mm. 

It  would  be  superfluous  to  describe  in  detail  the  behaviour 
of  the  10  remaining  cauterised  radicles.  The  corresponding 
control  specimens  all  became  geotropic  in  8  h.  Of  the  cauterised, 

6  were  first  looked  at  after  8  h.,  and  one  alone  showed  a  trace 


CHAP.  XI.       TRANSMITTED   EFFECTS  :   VICIA.  533 

of  geotropism  ;  4  were  first  looked  at  after  14  h.,  and  one  alone 
of  these  was  slightly  geotropic.  After  23-24  h.,  5  of  the  10  were 
still  horizontal,  4  slightly,  and  1  decidedly,  geotropic.  After 
48  h.  some  of  them  became  strongly  geotropic.  The  cauterised 
radicles  increased  greatly  in  length,  but  the  measurements  are 
not  worth  giving. 

As  five  of  the  last-mentioned  cauterised  radicles  had  become  in 
24  h.  somewhat  geotropic,  these  (together  with  three  which  were 
still  horizontal)  had  their  positions  reversed,  so  that  their  tips 
were  now  a  little  upturned,  and  they  were  again  touched  with 
caustic.  After  24  h.  they  showed  no  trace  of  geotropism ;  whereas 
the  eight  corresponding  control  specimens,  which  had  like- 
wise been  reversed,  in  which  position  the  tips  of  several  pointed 
to  the  zenith,  all  became  geotropic  ;  some  having  passed  in  the 
24  h.  through  an  angle  of  180°,  others  through  about  135°,  and 
others  through  only  90°.  The  eight  radicles,  which  had  been 
twice  cauterised,  were  observed  for  an  additional  day  (i.e.  for  48  h. 
after  being  reversed),  and  they  still  showed  no  signs  of  geotro- 
pism. Nevertheless,  they  continued  to  grow  rapidly ;  four  were 
measured  24  h.  after  being  reversed,  and  they  had  in  this  time 
increased  in  length  betweon  8  and  11  mm. ;  the  other  four  were 
measured  48  h.  after  being  reversed,  and  these  had  increased  by 
20, 18,  23,  and  28  mm. 

In  coming  to  a  conclusion  with  respect  to  the  effects  of  cauter- 
ising the  tips  of  these  radicles,  we  should  bear  in  mind, 
firstly,  that  horizontally  extended  control  radicles  were  always 
acted  on  by  geotropism,  and  became  somewhat  bowed  -down- 
wards in  8  or  9  h. ;  secondly,  that  the  chief  seat  of  the  curvature 
lies  at  a  distance  of  from  3  to  6  mm.  from  the  tip ;  thirdly,  that 
the  tip  was  discoloured  by  the  caustic  rarely  for  more  than 
1  mm.  in  length ;  fourthly,  that  the  greater  number  of  the  cau- 
terised radicles,  although  subjected  to  the  full  influence  of 
geotropism  during  the  whole  time,  remained  horizontal  for  24  h., 
and  some  for  twice  as  long ;  and  that  those  which  did  become 
bowed  were  so  only  in  a  slight  degree ;  fifthly,  that  the  cau- 
terised radicles  continued  to  grow  almost,  and  sometimes  quite, 
as  well  as  the  uninjured  ones  along  the  part  which  bends  most. 
And  lastly,  that  a  touch  on  the  tip  with  caustic,  if  on  one  side, 
far  from  preventing  curvature,  actually  induces  it.  Bearing  all 
these  facts  in  mind,  we  must  infer  that  under  normal  conditions 
the  geotropic  curvature  of  the  root  is  due  to  an  influence  trans- 
mitted from  the  apex  to  the  adjoining  part  where  the  bending 


534  SENSITIVENESS  TO   GRAVITATION.      CUAP.  XL 

takes  place ;  and  that  when  the  tip  of  the  root  is  cauterised  it  is 
unable  to  originate  the  stimulus  necessary  to  produce  geotropic 
curvature. 

As  we  had  observed  that  grease  was  highly  injurious  to  some 
plants,  we  determined  to  try  its  effects  on  radicles.  When  the 
cotyledons  of  Phalaris  and  Avena  were  covered  with  grease 
along  one  side,  the  growth  of  this  side  was  quite  stopped  or 
greatly  checked,  and  as  the  opposite  side  continued  to  grow,  the 
cotyledons  thus  treated  became  bowed  towards  the  greased  side. 
This  same  matter  quickly  killed  the  delicate  hypocotyls  and 
young  leaves  of  certain  plants.  The  grease  which  we  employed 
was  made  by  mixing  lamp-black  and  olive  oil  to  such  a  con- 
sistence that  it  could  be  laid  on  in  a  thick  layer.  The  tips  of 
five  radicles  of  the  bean  were  coated  with  it  for  a  length  of 
3  mm.,  and  to  our  surprise  this  part  increased  in  length  in  23  h. 
to  7 '  1  mm. ;  the  thick  layer  of  grease  being  curiously  drawn 
out.  It  thus  could  not  have  checked  much,  if  at  all,  the  growth 
of  the  terminal  part  of  the  radicle.  With  respect  to  geotropism, 
the  tips  of  seven  horizontally  extended  radicles  were  coated  for 
a  length  of  2  mm.,  and  after  24  h.  no  clear  difference  could  be 
perceived  between  their  downward  curvature  and  that  of  an 
equal  number  of  control  specimens.  The  tips  of  33  other  radicles 
were  coated  on  different  occasions  for  a  length  of  3  mm. ;  and 
they  were  compared  with  the  controls  after  8h.,  24  h.,  and  48  h. 
On  one  occasion,  after  24  h.,  there  was  very  little  difference  in 
curvature  between  the  greased  and  control  specimens;  but 
generally  the  difference  was  unmistakable,  those  with  greased 
tips  being  considerably  less  curved  downwards.  The  whole 
growing  part  (the  greased  tips  included)  of  six  of  these  radicles 
was  measured  and  was  found  to  have  increased  in  23  h.  from 
10  mm.  to  a  mean  length  of  17 '7  mm. ;  whilst  the  corresponding 
part  of  the  controls  had  increased  to  20'8  mm.  It  appears  there- 
fore, that  although  the  tip  itself,  when  greased,  continues  to 
grow,  yet  the  growth  of  the  whole  radicle  is  somewhat  checked, 
and  that  the  geotropic  curvature  of  the  upper  part,  which  was 
free  from  grease,  was  in  most  cases  considerably  lessened. 

Pisum  sativum. — Five  radicles,  extended  horizontally  over 
water,  had  their  tips  lightly  touched  two  or  three  times  with  dry 
caustic.  These  tips  were  measured  in  two  cases,  and  found  to 
be  blackened  for  a  length  of  only  half  a  millimeter.  Five  other 
radicles  were  left  as  controls.  The  part  which  is  most  bowed 
through  geotropism  lies  at  a  distance  of  several  millimeters  from 


CHAP.  XL     TBANSMJTTED   EFFECTS  :   PHASEOLUS.      535 

the  apex.  After  24  h.,  and  again  after  32  h.  from  the  commence- 
ment, four  of  the  cauterised  radicles  were  still  horizontal,  but 
one  was  plainly  geotropic,  being  inclined  at  45°  beneath  the 
horizon.  The  five  controls  were  somewhat  geotropic  after  7  h. 
20m.,  and  after  24  h.  were  all  strongly  geotropic ;  being  inclined 
at  the  following  angles  beneath  the  horizon,  viz.,  59°,  60°,  65°, 
57°,  and  43°.  The  length  of  the  radicles  was  not  measured  in 
either  set,  but  it  was  manifest  that  the  cauterised  radicles  had 
grown  greatly. 

The  following  case  proves  that  the  action  of  the  caustic  by 
itself  does  not  prevent  the  curvature  of  the  radicle.  Ten  radicles 
were  extended  horizontally  on  and  beneath  a  layer  of  damp 
friable  peat-earth;  and  before  being  extended  their  tips  were 
touched  with  dry  caustic  on  the  upper  side.  Ten  other  radicles 
similarly  placed  were  touched  on  the  lower  side ;  and  this  would 
tend  to  make  them  bend  from  the  cauterised  side ;  and  therefore, 
as  now  placed,  upwards,  or  in  opposition  to  geotropism.  Lastly, 
ten  uncauterised  radicles  were  extended  horizontally  as  controls. 
After  24  h.  all  the  latter  were  geotropic ;  and  the  ten  with  their 
tips  cauterised  on  the  upper  side  were  equally  geotropic ;  and 
we  believe  that  they  became  curved  downwards  before  the  con- 
trols. The  ten  which  had  been  cauterised  on  the  lower  side 
presented  a  widely  different  appearance :  No.  1,  however,  was 
perpendicularly  geotropic,  but  this  was  no  real  exception,  for  on 
examination  under  the  microscope,  there  was  no  vestige  of 
a  coloured  mark  on  the  tip,  and  it  was  evident  that  by  a  mistake 
it  had  not  been  touched  with  the  caustic.  No.  2  was  plainly 
geotropic,  being  inclined  at  about  45°  beneath  the  horizon;  No.  3 
was  slightly,  and  No.  4  only  just  perceptibly  geotropic;  Nos.  5 
and  6  were  strictly  horizontal ;  and  the  four  remaining  ones  were 
bowed  upwards,  in  opposition  to  geotropism.  In  these  four 
cases  the  radius  of  the  upward  curvatures  (according  to  Sachs' 
cyclometer)  was  5  mm.,  10  mm.,  30  mm.,  and  70  mm.  This  cur- 
vature was  distinct  long  before  the  24  h.  had  elapsed,  namely, 
after  8  h.  45  in.  from  the  time  when  the  lower  sides  of  the  tips 
were  touched  with  the  caustic. 

Phaseolus  multiflorus. — Eight  radicles,  serving  as  controls,  were 
extended  horizontally,  some  in  damp  friable  peat  and  some  in 
damp  air.  They  all  became  (temp.  20°-21°  C.)  plainly  geo- 
tropic in  8  h.  30  m.,  for  they  then  stood  at  an  average  angle  of  63° 
beneath  the  horizon.  A  rather  greater  length  of  the  radicle  is 
bowed  downwards  by  geotropism  than  in  the  case  of  Viciafaba, 


536  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XI. 

that  is  to  say,  rather  more  than  6mm.  as  measured  from  the  apex 
of  the  root-cap.  Nine  other  radicles  were  similarly  extended, 
three  in  damp  peat  and  six  in  damp  air,  and  dry  caustic  was 
held  transversely  to  their  tips  during  4  or  5  seconds.  Three  of 
their  tips  were  afterwards  examined :  in  (1)  a  length  of  0*  68  mm. 
was  discoloured,  of  which  the  basal  0'136  mm.  was  yellow,  the 
apical  part  being  black;  in  (2)  the  discoloration  was  0'65  mm. 
in  length,  of  which  the  basal  0*04  mm.  was  yellow ;  in  (3)  the  dis- 
coloration was  0 '  6  mm.  in  length,  of  which  the  basal  0'13  mm. 
was  yellow.  Therefore  less  than  1  mm.  was  affected  by  the  caustic, 
but  this  sufficed  almost  wholly  to  prevent  geotropic  action  ;  for 
after  24  h.  one  alone  of  the  nine  cauterised  radicles  became 
slightly  geotropic,  being  now  inclined  at  10°  beneath  the  horizon ; 
the  eight  others  remained  horizontal,  though  one  was  curved  a 
little  laterally. 

The  terminal  part  (10  mm.  in  length)  of  the  six  cauterised 
radicles  in  the  damp  air,  had  more  than  doubled  in  length  in 
the  24  h.,  for  this  part  was  now  on  an  average  20*7  mm.  long. 
The  increase  in  length  within  the  same  time  was  greater  in 
the  control  specimens,  for  the  terminal  part  had  grown  on  an 
average  from  10  mm.  to  26*6  mm.  But  as  the  cauterised 
radicles  had  more  than  doubled  their  length  in  the  24  h.,  it  is 
manifest  that  they  had  not  been  seriously  injured  by  the 
caustic.  We  may  here  add  that  when  experimenting  on  the 
effects  of  touching  one  side  of  the  tip  with  caustic,  too  much 
was  applied  at  first,  and  the  whole  tip  (but  we  believe  not  more 
than  1  mm.  in  length)  of  six  horizontally  extended  radicles  was 
killed,  and  these  continued  for  two  or  three  days  to  grow  out 
horizontally. 

Many  trials  were  made,  by  coating  the  tips  of  horizontally 
extended  radicles  with  the  before  described  thick  grease.  The 
geotropic  curvature  of  12  radicles,  which  were  thus  coated  for 
a  length  of  2  mm.,  was  delayed  during  the  first  8  or  9  h.,  but 
after  24  h.  was  nearly  as  great  as  that  of  the  control  speci- 
mens. The  tips  of  nine  radicles  were  coated  for  a  length  of  3  mm., 
and  after  7  h.  10  m.  these  stood  at  an  average  angle  of  30° 
beneath  the  horizon,  whilst  the  controls  stood  at  ao  average  of 
54°.  After  24  h.  the  two  lots  differed  but  little  in  their  degree 
of  curvature.  In  some  other  trials,  however,  there  was  a  fairly 
well-marked  difference  after  24  h.  between  those  with  greased 
tips  and  the  controls.  The  terminal  part  of  eight  control  speci- 
mens increased  in  24  h.  from  10  mm.  to  a  mean  length  of 


CHAP.  XL   TRANSMITTED  EFFECTS ;  CUCUKBITA.   537 

24 '3  mm..,  whilst  the  mean  increase  of  those  with  greased  tips 
was  20*7  mm.  The  grease,  therefore,  slightly  checked  the 
growth  of  the  terminal  part,  but  this  part  was  not  much 
injured;  for  several  radicles  which  had  been  greased  for  a 
length  of  2  mm.  continued  to  growr  during  seven  days,  and  were 
then  only  a  little  shorter  than  the  controls.  The  appearance 
presented  by  these  radicles  after  the  seven  days  was  very 
curious,  for  the  black  grease  had  been  drawn  out  into  the  finest 
longitudinal  striae,  with  dots  and  reticulations,  which  covered 
their  surfaces  for  a  length  of  from  26  to  44  mm.,  or  of  1  to 
1*7  inch.  We  may  therefore  conclude  that  grease  on  the  tips 
of  the  radicles  of  this  Phaseolus  somewhat  delays  and  lessens 
the  geotropic  curvature  of  the  part  which  ought  to  bend 
most. 

Gossypium  herbaceum. — The  raiicles  of  this  plant  bend, 
through  the  action  of  geotropism,  for  a  length  of  about  6  mm. 
Five  radicles,  placed  horizontally  in  damp  air,  had  their  tips 
touched  with  caustic,  and  the  discoloration  extended  for  a 
length  of  from  f  to  1  mm.  They  showed,  after  7  h.  45  m.  and 
again  after  23  h.,  not  a  trace  of  geotropism ;  yet  the  terminal 
portion,  9  mm.  in  length,  had  increased  on  an  average  to 
15 '9  mm.  Six  control  radicles,  after  7  h.  45  m.,  were  all  plainly 
geotropic,  two  of  them  being  vertically  dependent,  and  after 
23  h.  all  were  vertical,  or  nearly  so. 

Cucurbita  ovifera. — A  large  number  of  trials  proved  almost 
useless,  from  the  three  following  causes:  Firstly,  the  tips  of 
radicles  which  have  grown  somewhat  old  are  only  feebly  geo- 
tropic if  kept  in  damp  air ;  nor  did  we  succeed  well  in  our 
experiments,  until  the  germinating  seeds  were  placed  in  peat 
and  kept  at  a  rather  high  temperature.  Secondly,  the  hypocotyls 
of  the  seeds  which  were  pinned  to  the  lids  of  the  jars  gradually 
became  arched ;  and,  as  the  cotyledons  were  fixed,  the  movement 
of  the  hypocotyl  affected  the  position  of  the  radicle,  and  caused 
confusion.  Thirdly,  the  point  of  the  radicle  is  so  fine  that  it  is 
difficult  not  to  cauterise  it  either  too  much  or  too  little.  But 
we  managed  generally  to  overcome  this  latter  difficulty,  as  the 
following  experiments  show,  which  are  given  to  prove  that  a 
touch  with  caustic  on  one  side  of  the  tip  does  not  prevent  the 
upper  part  of  the  radicle  from  bending.  Ten  radicles  were  laid 
horizontally  beneath  and  on  damp  friable  peat,  and  their  tips 
were  touched  with  caustic  on  the  upper  side.  After  8  h.  all 
were  plainly  geotropic,  three  of  them  rectangularly;  after  19  h. 


538  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XT. 

all  were  strongly  geotropic,  most  of  them  pointing  perpen- 
dicularly downwards.  Ten  other  radicles,  similarly  placed,  had 
their  tips  touched  with  caustic  on  the  lower  side;  after  8  h. 
three  were  slightly  geotropic,  but  not  nearly  so  much  so  as  the 
least  geotropic  of  the  foregoing  specimens ;  four  remained  hori- 
zontal; and  three  were  curved  upwards  in  opposition  to  geo- 
tropism.  After  19  h.  the  three  which  were  slightly  geotropic 
had  become  strongly  so.  Of  the  four  horizontal  radicles,  one 
alone  showed  a  trace  of  geotropism;  of  the  three  up-curved 
radicles,  one  retained  this  curvature,  and  the  other  two  had 
become  horizontal. 

The  radicles  of  this  plant,  as  already  remarked,  do  not  succeed 
well  in  damp  air,  but  the  result  of  one  trial  may  be  briefly 
given.  Nine  young  radicles  between  '3  and  -5  inch  in  length, 
with  their  tips  cauterised  and  blackened  for  a  length  never 
exceeding  £  mm.,  together  with  eight  control  specimens,  were 
extended  horizontally  in  damp  air.  After  an  interval  of  only 

4  h.  10  m.  all  the  controls  were  slightly  geotropic,  whilst  not 
one  of  the  cauterised  specimens  exhibited  a  trace  of  this  action. 
After  8  h.  35  m.,  there  was  the  same  difference  between  the 
two  sets,  but  rather  more  strongly  marked.    By  this  time  both 
sets  had  increased  greatly  in  length.     The  controls,  however, 
never  became  much  more  curved  downwards ;  and  after  24  h. 
there  was  no  great  difference  between  the  two  sets  in  their 
degree  of  curvature. 

Eight  young  radicles  of  nearly  equal  length  (average  '36  inch) 
were  placed  beneath  and  on  peat-earth,  and  were  exposed  to  a 
temp,  of  75°-76°  F.  Their  tips  had  been  touched  transversely 
with  caustic,  and  five  of  them  were  blackened  for  a  length  of 
about  0*5  mm.,  whilst  the  other  three  were  only  just  visibly  dis- 
coloured. In  the  same  box  there  were  15  control  radicles,  mostly 
about  *36  inch  in  length,  but  some  rather  longer  and  older,  and 
therefore  less  sensitive.  After  5  h.,  the  15  control  radicles  were 
all  more  or  less  geotropic  :  after  9  h.,  eight  of  them  were  bent 
down  beneath  the  horizon  at  various  angles  between  45°  and  90°, 
the  remaining  seven  being  only  slightly  geotropic :  after  25  h.  all 
were  rectangularly  geotropic.  The  state  of  the  eight  cauterised 
radicles  after  the  same  intervals  of  time  was  as  follows:  after 

5  h.  one  alone  was  slightly  geotropic,  and  this  was  one  with 
the  tip  only  a  very  little  discoloured:  after  9  h.  the  one  just 
mentioned  was  rectangularly  geotropic,  and  two  others  were 
slightly  so,  and  these  were  the  three  which  had  been  scarcely 


CHAP.  XI.  TRANSMITTED   EFFECTS  :   ZEA.  539 

affected  by  the  caustic ;  the  other  five  were  still  strictly  hori- 
zontal. After  24  h.  40  m.  the  three  with  only  slightly  discoloured 
tips  were  bent  down  rectangularly;  the  other  five  were  not  in 
the  least  affected,  but  several  of  them  had  grown  rather  tor- 
tuously, though  still  in  a  horizontal  plane.  The  eight  cauterised 
radicles  which  had  at  first  a  mean  length  of  '36  inch,  after  9  h. 
had  increased  to  a  mean  length  of  '79  inch ;  and  after  24  h. 
40  m.  to  the  extraordinary  mean  length  of  2  inches.  There 
was  no  plain  difference  in  length  between  the  five  well  cau- 
terised radicles  which  remained  horizontal,  and  the  three  with 
slightly  cauterised  tips  which  had  become  abruptly  bent  down. 
A  few  of  the  control  radicles  were  measured  after  25  h.,  and 
they  were  on  an  average  only  a  little  longer  than  the  cauterised, 
viz.,  2'19  inches.  We  thus  see  that  killing  the  extreme  tip  of 
the  radicle  of  this  plant  for  a  length  of  about  0'5  mm.,  though  it 
stops  the  geotropic  bending  of  the  upper  part,  hardly  interferes 
with  the  growth  of  the  whole  radicle. 

In  the  same  box  with  the  15  control  specimens,  the  rapid  geo- 
tropic bending  and  growth  of  which  have  just  been  described, 
there  were  six  radicles,  about  '6  inch  in  length,  extended  hori- 
zontally, from  which  the  tips  had  been  cut  off  in  a  transverse 
direction  for  a  length  of  barely  1  mm.  These  radicles  were 
examined  after  9  h.  and  again  after  24  h.  40  m.,  and  they  all 
remained  horizontal.  They  had  not  become  nearly  so  tortuous 
as  those  above  described  which  had  been  cauterised.  The 
radicles  with  their  tips  cut  off  had  grown  in  the  24  h.  40  m.  as 
much,  judging  by  the  eye,  as  the  cauterised  specimens. 

Zea  mays. — The  tips  of  several  radicles,  extended  horizontally 
in  damp  air,  were  dried  with  blotting-paper  and  then  touched 
in  the  first  trial  during  2  or  3  seconds  with  dry  caustic ;  but 
this  was  too  long  a  contact,  for  the  tips  were  blackened  for  a 
length  of  rather  above  1  mm.  They  showed  no  signs  of  geo- 
tropism  after  an  interval  of  9  h.,  and  were  then  thrown  away. 
In  a  second  trial  the  tips  of  three  radicles  were  touched  for  a 
shorter  time,  and  were  blackened  for  a  length  of  from  0'5  to 
0'75  mm. :  they  all  remained  horizontal  for  4  h.,  but  after  8  h. 
30  m.  one  of  them,  in  which  the  blackened  tip  was  only  0-5  mm. 
in  length,  was  inclined  at  21°  beneath  the  horizon.  Six  con- 
trol radicles  all  became  slightly  geotropic  in  4  h.,  and  strongly 
so  after  8  h.  30  m.,  with  the  chief  seat  of  curvature  generally 
between  6  or  7  mm.  from  the  apex.  In  the  cauterised  specimens, 
the  terminal  growing  part,  10  mm.  in  length,  increased  during 


540  SENSITIVENESS   TO  GRAVITATION.      CHAP.  XL 

the  8  h.  30  m.  to  a  mean  length  of  13  inin. ;  and  in  the  controls 
to  14  3  mm. 

In  a  third  trial  the  tips  of  five  radicles  (exposed  to  a  temp, 
of  70°-71°)  were  touched  with  the  caustic  only  once  and  very 
slightly  ;  they  were  afterwards  examined  under  the  microscope, 
and  the  part  which  was  in  any  way  discoloured  was  on  an 
average  '76  mm.  in  length.  After  4  h.  10  m.  none  were  bent ; 
after  5  h.  45  m.,  and  again  after  23  h.  30  m.,  they  still  remained 
horizontal,  excepting  one  which  was  now  inclined  20°  beneath 
the  horizon.  The  terminal  part,  10  mm.  in  length,  had  in- 
creased greatly  in  length  during  the  23  h.  30  m.,  viz.,  to  an 
average  of  26  mm.  Four  control  radicles  became  slightly  geo- 
tropic  after  the  4  h.  10  m.,  and  plainly  so  after  the  5  h.  45  m. 
Their  mean  length  after  the  23  h.  30  m.  had  increased  from 
10  mm.  to  31  mm.  Therefore  a  slight  cauterisation  of  the  tip 
checks  slightly  the  growth  of  the  whole  radicle,  and  manifestly 
stops  the  bending  of  that  part  which  ought  to  bend  most  under 
the  influence  of  geotropism  and  which  still  continues  to 
increase  greatly  in  length. 


Concluding  Remarks. — Abundant  evidence  has  now 
been  given,  showing  that  with  various  plants  the  tip 
of  the  radicle  is  alone  sensitive  to  geotropism ;  and 
that  when  thus  excited,  it  causes  the  adjoining  parts 
to  bend.  The  exact  length  of  the  sensitive  part  seems 
to  be  somewhat  variable,  depending  in  part  on  the  age 
of  the  radicle ;  but  the  destruction  of  a  length  of  from 
less  than  1  to  1'5  mm.  (about  -^th  of  an  inch),  in  the 
several  species  observed,  generally  sufficed  to  prevent 
any^part  of  the  radicle  from  bending  within  24  h.,  or 
even  for  a  longer  period.  The  fact  of  the  tip  alone 
being  sensitive  is  so  remarkable  a  fact,  that  we  will 
here  give  a  brief  summary  of  the  foregoing  experiments. 
The  tips  were  cut  off  29  horizontally  extended  radicles 
of  Vicia  faba,  and  with  a  few  exceptions  they  did  not 
become  geotropic  in  22  or  23  h.,  whilst  unmutilated 
radicles  were  always  bowed  downwards  in  8  or  9  h.  It 
should  be  borne  in  mind  that  the  mere  act  of  cutting 


CHAP.  XL     TRANSMITTED  EFFECTS  :    CONCLUSION.     5<il 

off  the  tip  of  a  horizontally  extended  radicle  does  not 
prevent  the  adjoining  parts  from  bending,  if  the  tip 
has  been  previously  exposed  for  an  hour  or  two  to  the 
influence  of  geotropism.  The  tip  after  amputation  is 
sometimes  completely  regenerated  in  three  days ;  and 
it  is  possible  that  it  may  be  able  to  transmit  an 
impulse  to  the  adjoining  parts  before  its  complete 
regeneration.  The  tips  of  six  radicles  of  Cucurbita 
ovifera  were  amputated  like  those  of  Vicia  faba  ;  and 
these  radicles  showed  no  signs  of  geotropism  in  24  h.  ; 
whereas  the  control  specimens  were  slightly  affected 
in  5  h.,  and  strongly  in  9  h. 

With  plants  belonging  to  six  genera,  the  tips  of  the 
radicles  were  touched  transversely  with  dry  caustic ; 
and  the  injury  thus  caused  rarely  extended  for  a  greater 
length  than  1  mm.,  and  sometimes  to  a  less  distance,  as 
judged  by  even  the  faintest  discoloration.  We  thought 
that  this  would  be  a  better  method  of  destroying  the 
vegetative  point  than  cutting  it  off ;  for  we  knew,  from 
many  previous  experiments  and  from  some  given  in 
the  present  chapter,  that  a  touch  with  caustic  on  one 
side  of  the  apex,  far  from  preventing  the  adjoining 
part  from  bending,  caused  it  to  bend.  In  all  the 
following  cases,  radicles  with  uncauterised  tips  were 
observed  at  the  same  time  and  under  similar  circum- 
stances, and  they  became,  in  almost  every  instance, 
plainly  bowed  downwards  in  one-half  or  one-third  of 
the  time  during  which  the  cauterised  specimens  were 
observed.  With  Vicia  faba  19  radicles  were  cau- 
terised ;  12  remained  horizontal  during  23-24  h. ; 
6  became  slightly  and  1  strongly  geotropic.  Eight  of 
these  radicles  were  afterwards  reversed,  and  again 
touched  with  caustic,  and  none  of  them  became  geo- 
tropic in  24  h.,  whilst  the  reversed  control  specimens 
became  strongly  bowed  downwards  within  this  time. 


542  SENSITIVENESS   TO   GRAVITATION.      CHAP.  XI. 

With  Pisum  sativum,  five  radicles  had  their  tips  touched 
with  caustic,  and  after  32  h.  four  were  still  horizontal. 
The  control  specimens  were  slightly  geotropic  in 
7  h.  20  m.,  and  strongly  so  in  24  h.  The  tips  of  9  other 
radicles  of  this  plant  were  touched  only  on  the  lower 
side,  and  6  of  them  remained  horizontal  for  24  h.,  or 
were  upturned  in  opposition  to  geotropism  ;  2  were 
slightly,  and  1  plainly  geotropic.  With  Phaseolus 
multiflorus,  15  radicles  were  cauterised,  and  8  re- 
mained horizontal  for  24  h. ;  whereas  all  the  controls 
were  plainly  geotropic  in  8  h.  30  m.  Of  5  cauterised 
radicles  of  Gossypium  herbaceum,  4  remained  horizontal 
for  23  h.  and  1  became  slightly  geotropic ;  6  control 
radicles  were  distinctly  geotropic  in  7  h.  45  m.  Five 
radicles  of  Cucurbita  ovifera  remained  horizontal  in 
peat-earth  during  25  h.,  and  9  remained  so  in  damp 
air  during  8  J  h. ;  whilst  the  controls  became  slightly 
geotropic  in  4  h.  10  m.  The  tips  of  10  radicals  of  this 
plant  were  touched  on  their  lower  sides,  and  6  of 
them  remained  horizontal  or  were  upturned  after  19  h., 
1  being  slightly  and  3  strongly  geotropic. 

Lastly,  the  tips  of  several  radicles  of  Vic.ia.faba  and 
Phaseolus  multiflorus  were  thickly  coated  with  grease 
for  a  length  of  3  mm.  This  matter,  which  is  highly 
injurious  to  most  plants,  did  not  kill  or  stop  the  growth 
of  the  tips,  and  only  slightly  lessened  the  rate  of 
growth  of  the  whole  radicle  ;  but  it  generally  delayed 
a  little  the  geotropic  bending  of  the  upper  part. 

The  several  foregoing  cases  would  tell  us  nothing, 
if  the  tip  itself  was  the  part  which  became  most 
bent ;  but  we  know  that  it  is  a  part  distant  from  the 
tip  by  some  millimeters  which  grows  quickest,  and 
which,  under  the  influence  of  geotropism,  bends  most. 
We  have  110  reason  ito  suppose  that  this  part  is  injured 
by  the  death  or  injury  of  the  tip ;  and  it  is  certain 


CHAP.  XI.     TKANSMITTED  EFFECTS  :   CONCLUSION.    513 

that  after  the  tip  has  been  destroyed  this  part  goes  on 
growing  at  such  a  rate,  that  its  length  was  often  doubled 
in  a  day.  We  have  also  seen  that  the  destruction  of  the 
tip  does  not  prevent  the  adjoining  part  from  bending, 
if  this  part  has  already  received  some  influence  from 
the  tip.  As  with  horizontally  extended  radicles,  of 
which  the  tip  has  been  cut  off  or  destroyed,  the  part 
which  ought  to  bend  most  remains  motionless  for 
many  hours  or  days,  although  exposed  at  right  angles 
to  the  full  influence  of  geotropism,  we  must  conclude 
that  the  tip  alone  is  sensitive  to  this  power,  and  trans- 
mits some  influence  or  stimulus  to  the  adjoining  parts, 
causing  them  to  bend.  We  have  direct  evidence  of 
such  transmission ;  for  when  a  radicle  was  left  extended 
horizontally  for  an  hour  or  an  hour  and  a  half,  by 
which  time  the  supposed  influence  will  have  travelled 
a  little  distance  from  the  tip,  and  the  tip  was  then 
cut  off,  the  radicle  afterwards  became  bent,  although 
placed  perpendicularly.  The  terminal  portions  of 
several  radicles  thus  treated  continued  for  some  time 
to  grow  in  the  direction  of  their  newly-acquired  curva- 
ture ;  for  as  they  were  destitute  of  tips,  they  were  no 
longer  acted  on  by  geotropism.  But  after  three  or 
four  days  when  new  vegetative  points  were  formed,  the 
radicles  were  again  acted  on  by  geotropism,  and  now 
they  curved  themselves  perpendicularly  downwards. 
To  see  anything  of  the  above  kind  in  the  animal 
kingdom,  we  should  have  to  suppose  that  an  animal 
whilst  lying  down  determined  to  rise  up  in  some  par- 
ticular direction ;  and  that  after  its  head  had  been  cut 
off,  an  impulse  continued  to  travel  very  slowly  along 
the  nerves  to  the  proper  muscles  ;  so  that  after  several 
hours  the  headless  animal  rose  up  in  the  predeter- 
mined direction. 

As  tho  tip  of  the  radicle  has  been  found  to  be  the 
24 


544  SENSITIVENESS   TO   GKAVITATION.     CHAP.  XI. 

part  which  is  sensitive  to  geotropism  in  the  members  of 
such  distinct  families  as  the  Leguminosse,  Malvaceae, 
Cucurbitacese  and  Gramineae,  we  may  infer  that  this 
character  is  common  to  the  roots  of  most  seedling 
plants.  Whilst  a  root  is  penetrating  the  ground,  the 
tip  must  travel  first ;  and  we  can  see  the  advantage  of 
its  being  sensitive  to  geotropism,  as  it  has  to  deter- 
mine the  course  of  the  whole  root.  Whenever  the  tip 
is  deflected  by  any  subterranean  obstacle,  it  will  also 
be  an  advantage  that  a  considerable  length  of  the  root 
should  be  able  to  bend,  more  especially  as  the  tip 
itself  grows  slowly  and  bends  but  little,  so  that  the 
proper  downward  course  may  be  soon  recovered.  But 
it  appears  at  first  sight  immaterial  whether  this  were 
effected  by  the  whole  growing  part  being  sensitive  to 
geotropism,  or  by  an  influence  transmitted  exclusively 
from  the  tip.  We  should,  however,  remember  that  it 
is  the  tip  which  is  sensitive  to  the  contact  of  hard 
objects,  causing  the  radicle  to  bend  away  from  them, 
thus  guiding  it  along  the  lines  of  least  resistance  in 
the  soil.  It  is  again  the  tip  which  is  alone  sensitive, 
at  least  in  some  cases,  to  moisture,  causing  the 
radicle  to  bend  towards  its  source.  These  two  kinds 
of  sensitiveness  conquer  for  a  time  the  sensitiveness 
to  geotropism,  which,  however,  ultimately  prevails. 
Therefore,  the  three  kinds  of  sensitiveness  must  often 
come  into  antagonism  ;  first  one  prevailing,  and  then 
another ;  and  it  would  be  an  advantage,  perhaps  a 
necessity,  for  the  interweighing  and  reconciling  of 
these  three  kinds  of  sensitiveness,  that  they  should 
be  all  localised  in  the  same  group  of  cells  which  have 
to  transmit  the  command  to  the  adjoining  parts  of 
the  radicle,  causing  it  to  bend  to  or  from  the  source  of 
irritation. 

Finally,  the  fact  of  the  tip  alone  being  sensitive  to 


CIIAP.  XL    TRANSMITTED   EFFECTS:   CONCLUSION.     545 

the  attraction  of  gravity  has  an  important  bearing  on 
the  theory  of  geotropism.  Authors  seem  generally  to 
look  at  the  bending  of  a  radicle  towards  the  centre  of 
the  earth,  as  the  direct  result  of  gravitation,  which  is 
believed  to  modify  the  growth  of  the  upper  or  lower 
surfaces,  in  such  a  manner  as  to  induce  curvature  in 
the  proper  direction.  But  we  now  know  that  it  is  the 
tip  alone  which  is  acted  on,  and  that  this  part  trans- 
mits some  influence  to  the  adjoining  parts,  causing 
them  to  curve  downwards.  Gravity  does  not  appear 
to  act  in  a  more  direct  manner  on  a  radicle,  than  it 
does  on  any  lowly  organised  animal,  which  moves 
away  when  it  feels  some  weight  or  pressure. 


546  SUMMARY   AND  CHAP.  XII. 


CHAPTER  XII. 

SUMMARY   AND    CONCLUDING    KEMARKS. 

Nature  of  the  circumnutating  movement — History  of  a  germinating 
seed  —  The  radicle  first  protrudes  and  circumnutates — Its  tip 
highly  sensitive — Emergence  of  the  hypocotyl  or  of  the  epicotyl 
from  the  ground  under  the  form  of  an  arch — Its  circumnutation 
and  that  of  the  cotyledons — The  seedling  throws  up  a  leaf-bearing 
stem — The  circumnutation  of  all  the  parts  or  organs— Modified 
circumnutation — Epinasty  and  hyponasty — Movements  of  climbing 
plants  — Nyctitropic  movements — Movements  excited  by  light  and 
gravitation  —  Localised  sensitiveness  —  Resemblance  between  the 
movements  of  plants  and  animals — The  tip  of  the  radicle  acts  like 
a  brain. 

IT  may  be  useful  to  the  reader  if  we  briefly  sum  up 
the  chief  conclusions,  which,  as  far  as  we  can  judge, 
have  been  fairly  well  established  by  the  observations 
given  in  this  volume.  All  the  parts  or  organs  in 
every  plant  whilst  they  continue  to  grow^  and  some 
parts  which  are  provided  with  pulvini  after  they  have 
ceased  to  grow,  are  continually  circumnutating.  This 
movement  commences  even  before  the  young  seedling 
has  broken  through  the  ground.  The  nature  of  the 
movement  and  its  causes,  as  far  as  ascertained,  have 
been  briefly  described  in  the  Introduction.  Why 
every  part  of  a  plant  whilst  it  is  growing,  and  in  some 
cases  after  growth  has  ceased,  should  have  its  cells 
rendered  more  turgescent  and  its  cell-walls  more 
extensile  first  on  one  side  and  then  on  another,  thus 
inducing  circumnutation,  is  not  known.  It  would 
appear  as  if  the  changes  in  the  cells  required  periods 
of  rest. 


CHAP.  XII.  CONCLUDING   REMARKS.  547 

In  some  cases,  as  with  the  hypocotyls  of  Brassica, 
the  leaves  of  Dionsea  and  the  joints  of  the  Graminerc, 
the  circumnutating  movement  when  viewed  under  the 
microscope  is  seen  to  consist  of  innumerable  small 
oscillations.  The  part  under  observation  suddenly 
jerks  forwards  for  a  length  of  '002  to  *001  of  an  inch, 
and  then  slowly  retreats  for  a  part  of  this  distance ; 
after  a  few  seconds  it  again  jerks  forwards,  but  with 
many  intermissions.  The  retreating  movement  appa- 
•  rently  is  due  to  the  elasticity  of  the  resisting  tissues. 
How  far  this  oscillatory  movement  is  general  we  do 
not  know,  as  not  many  circumnutating  plants  were 
observed  by  us  under  the  microscope  ;  but  no  such 
movement  could  be  detected  in  the  case  of  Drosera 
with  a  2-inch  object-glass  which  we  used.  The  pheno- 
menon is  a  remarkable  one.  The  whole  hypocotyl 
of  a  cabbage  or  the  whole  leaf  of  a  Dionaea  could  not 
jerk  forwards  unless  a  very  large  number  of  cells  on 
one  side  were  simultaneously  affected.  Are  we  to  sup- 
pose that  these  cells  steadily  become  more  and  more 
turgescent  on  one  side,  until  the  part  suddenly  yields 
and  bends,  inducing  what  may  be  called  a  micro- 
scopically minute  earthquake  in  the  plant ;  or  do  the 
cells  on  one  side  suddenly  become  turgescent  in  an 
intermittent  manner ;  each  forward  movement  thus 
caused  being  opposed  by  the  elasticity  of  the  tissues  ? 

Circumnutation  is  of  paramount  importance  in  the 
life  of  every  plant ;  for  it  is  through  its  modification 
that  many  highly  beneficial  or  necessary  movements 
have  been  acquired.  When  light  strikes  one  side 
of  a  plant,  or  light  changes  into  darkness,  or  when 
gravitation  acts  on  a  displaced  part,  the  plant  is 
enabled  in  some  unknown  manner  to  increase  the 
always  varying  turgescence  of  the  cells  on  one  side ; 
so  that  the  ordinary  circumnutating  movement  is 


548  SUMMARY  AND  CHAP.  XII. 

modified,  and  the  part  bends  either  to  or  from  the 
exciting  cause ;  or  it  may  occupy  a  new  position,  as 
in  the  so-called  sleep  of  leaves.  The  influence  which 
modifies  circuninutation  may  be  transmitted  from  one 
part  to  another.  Innate  or  constitutional  changes, 
independently  of  any  external  agency,  often  modify 
the  circumnutating  movements  at  particular  periods 
of  the  life  of  the  plant.  As  circumnutation  is  uni- 
versally present,  we  can  understand  how  it  is  that 
movements  of  the  same  kind  have  been  developed  in 
the  most  distinct  members  of  the  vegetable  series. 
But  it  must  not  be  supposed  that  all  the  movements 
of  plants  arise  from  modified  circumnutation ;  for,  as 
we  shall  presently  see,  there  is  reason  to  believe  that 
this  is  not  the  case. 

Having  made  these  few  preliminary  remarks,  we 
will  in  imagination  take  a  germinating  seed,  and  con- 
sider the  part  which  the  various  movements  play  in 
the  life-history  of  the  plant.  The  first  change  is  the 
protrusion  of  the  radicle,  which  begins  at  once  to 
circumnutate.  This  movement  is  immediately  modi- 
fied by  the  attraction  of  gravity  and  rendered  geo- 
tropic.  The  radicle,  therefore,  supposing  the  seed  to 
be  lying  on  the  surface,  quickly  bends  downwards,  fol- 
lowing a  more  or  less  spiral  course,  as  was  seen  on  the 
smoked  glass-plates.  Sensitiveness  to  gravitation  re- 
sides in  the  tip;  and  it  is  the  tip  which  transmits 
some  influence  to  the  adjoining  parts,  causing  them 
to  bend.  As  soon  as  the  tip,  protected  by  the  root- 
cap,  reaches  the  ground,  it  penetrates  the  surface,  if 
this  be  soft. or  friable  ;  and  the  act  of  penetration  is 
apparently  aided  by  the  rocking  or  circumnutating 
movement  of  the  whole  end  of  the  radicle.  If  the  sur- 
face is  compact,  and  cannot  easily  be  penetrated,  then 


CHAP.  XII.  CONCLUDING   REMARKS.  549 

the  seed  itself,  unless  it  be  a  heavy  one,  is  displaced 
or  lifted  up  by  the  continued  growth  and  elongation 
of  the  radicle.  But  in  a  state  of  nature  seeds  often 
get  covered  with  earth  or  other  matter,  or  fall  into 
crevices,  &c.,  and  thus  a  point  o£  resistance  is  afforded, 
and  the  tip  can  more  easily  penetrate  the  ground. 
But  even  with  seeds  lying  loose  on  the  surface  there 
is  another  aid  :  a  multitude  of  excessively  fine  hairs 
are  emitted  from  the  upper  part  of  the  radicle,  and 
these  attach  themselves  firmly  to  stones  or  other  ob- 
jects lying  on  the  surface,  and  can  do  so  even  to  glass ; 
and  thus  the  upper  part  is  held  down  whilst  the  tip 
presses  against  and  penetrates  the  ground.  The 
attachment  of  the  root-hairs  is  effected  by  the  lique- 
faction of  the  outer  surface  of  the  cellulose  walls,  and 
by  the  subsequent  setting  hard  of  the  liquefied  matter. 
This  curious  process  probably  takes  place,  not  for 
the  sake  of  the  attachment  of  the  radicles  to  superficial 
objects,  but  in  order  that  the  hairs  may  be  brought  into 
the  closest  contact  with  the  particles  in  the  soil,  by 
which  means  they  can  absorb  the  layer  of  water  sur- 
rounding them,  together  with  any  dissolved  matter. 

After  the  tip  has  penetrated  the  ground  to  a  little 
depth,  the  increasing  thickness  of  the  radicle,  together 
with  the  root-hairs,  hold  it  securely  in  its  place ;  and 
now  the  force  exerted  by  the  longitudinal  growth  of 
the  radicle  drives  the  tip  deeper  into  the  ground. 
This  force,  combined  with  that  due  to  transverse 
growth,  gives  to  the  radicle  the  power  of  a  wedge. 
Even  a  growing  root  of  moderate  size,  such  as  that 
of  a  seedling  bean,  can  displace  a  weight  of  some 
pounds.  It  is  not  probable  that  the  tip  when  buried 
in  compact  earth  can  actually  circurnnutate  and  thus 
aid  its  downward  passage,  but  the  circumnutating 
movement  will  facilitate  the  tip  entering  any  lateral 


550  SUMMARY  AND  CHAP.  XII. 

or  oblique  fissure  in  the  earth,  or  a  burrow  made  by 
an  earth-worm  or  larva;  and  it  is  certain  that  roots 
often  run  down  the  old  burrows  of  worms.  The  tip, 
however,  in  endeavouring  to  circumnutate,  will  con- 
tinually press  against  the  earth  on  all  sides,  and  this 
can  hardly  fail  to  be  of  the  highest  importance  to  the 
plant ;  for  we  have  seen  that  when  little  bits  of  card- 
like  paper  and  of  very  thin  paper  were  cemented  on 
opposite  sides  of  the  tip,  the  whole  growing  part  of 
the  radicle  was  excited  to  bend  away  from  the  side 
bearing  the  card  or  more  resisting  substance,  towards 
the  side  bearing. the  thin  paper.  We  may  therefore 
feel  almost  sure  that  when  the  tip  encounters  a  stone 
or  other  obstacle  in  the  ground,  or  even  earth  more 
compact  on  one  side  than  the  other,  the  root  will  bend 
away  as  much  as  it  can  from  the  obstacle  or  the  more 
resisting  earth,  and  will  thus  follow  with  unerring 
skill  a  line  of  least  resistance. 

The  tip  is  more  sensitive  to  prolonged  contact  with 
an  object  than  to  gravitation  when  this  acts  obliquely 
on  the  radicle,  and  sometimes  even  when  it  acts  in  the 
most  favourable  direction  at  right  angles  to  the  radicle. 
The  tip  was  excited  by  an  attached  bead  of  shellac, 
weighing  less  than  ^Jo^  °^  a  grain  (0*33  mg.) ;  it  is 
therefore  more  sensitive  than  the  most  delicate  ten- 
dril, namely,  that  of  Passiflora  gracilis,  which  was  barely 
acted  on  by  a  bit  of  wire  weighing  s^th  of  a  grain.  But 
this  degree  of  sensitiveness  is  as  nothing  compared  with 
that  of  the  glands  of  Drosera,  for  these  are  excited  by 
particles  weighing  only  y^o  °f  a  grain-  The  sensi- 
tiveness of  the  tip  cannot  be  accounted  for  by  its 
being  covered  by  a  thinner  layer  of  tissue  than  the 
other  parts,  for  it  is  protected  by  the  relatively  thick 
root-cap.  It  is  remarkable  that  although  the  radicle 
bends  away,  when  one  side  of  the  tip  is  slightly  touched 


CHAP.  XII.          *   CONCLUDING  KEMAKKS.  551 

with  caustic,  yet  if  the  side  be  much  cauterised  the 
injury  is  too  great,  and  the  power  of  transmitting  some 
influence  to  the  adjoining  parts  causing  them  to  bend, 
is  lost.  Other  analogous  cases  are  known  to  occur. 

After  a  radicle  has  been  deflected  by  some  obstacle, 
geotropism  directs  the  tip  again  to  grow  perpendicu- 
larly downwards ;  but  geotropism  is  a  feeble  power, 
and  here,  as  Sachs  has  shown,  another  interesting 
adaptive  movement  comes  into  play ;  for  radicles  at 
a  distance  of  a  few  millimeters  from  the  tip  are 
sensitive  to  prolonged  contact  in  such  a  manner  that 
they  bend  towards  the  touching  object,  instead  of  from 
it  as  occurs  when  an  object  touches  one  side  of  the 
tip.  Moreover,  the  curvature  thus  caused  is  abrupt; 
the  pressed  part  alone  bending.  Even  slight  pressure 
suffices,  such  as  a  bit  of  card  cemented  to  one  side. 
Therefore  a  radicle,  as  it  psssc-s  over  the  edge  of  any 
obstacle  in  the  ground,  will  through  the  action  of  gee* 
tropism  press  against  it ;  and  this  pressure  will  cause 
the  radicle  to  endeavour  to  bend  abruptly  over  the 
edge,,  It  will  thus  recover  as  quickly  as  possible  its 
normal  downward  course. 

Radicles  are  also  sensitive  to  air  which  contains 
more  moisture  on  one  side  than  the  other,  and  they 
bend  towards  its  source.  It  is  therefore  probable  that 
they  are  in  like  manner  sensitive  to  dampness  in  the 
soil.  It  was  ascertained  in  several  cases  that  this 
sensitiveness  resides  in  the  tip,  which  transmits  an 
influence  causing  the  adjoining  upper  part  to  bend 
in  opposition  to  geotropism  towards  the  moist  object. 
We  may  therefore  infer  that  roots  will  be  deflected 
from  their  downward  course  towards  any  source  of 
moisture  in  the  soil. 

Again,  most  or  all  radicles  are  slightly  sensitive  to 
light,  and,  according  to  Wiesner,  generally  bend  a  little 


552  SUMMARY  AND  CHAP.  XII. 

from  it.  Whether  this  can  be  of  any  service  to  them 
is  very  doubtful,  but  with  seeds  germinating  on  the 
surface  it  will  slightly  aid  geotropism  in  directing 
the  radicles  to  the  ground.*  We  ascertained  in  one 
instance  that  such  sensitiveness  resided  in  the  tip,  and 
caused  the  adjoining  parts  to  bend  from  the  light. 
The  sub-aerial  roots  observed  by  Wiesner  were  all 
apheliotropic,  and  this,  no  doubt,  is  of  use  in  bringing 
them  into  contact  with  trunks  of  trees  or  surfaces  of 
rock,  as  is  their  habit. 

We  thus  see  that  with  seedling  plants  the  tip  of  the 
radicle  is  endowed  with  diverse  kinds  of  sensitiveness  ; 
and  that  the  tip  directs  the  adjoining  growing  parts 
to  bend  to  or  from  the  exciting  cause,  according  to  the 
needs  of  the  plant.  The  sides  of  the  radicle  are  also 
sensitive  to  contact,  but  in  a,  widely  different  manner. 
Gravitation,  though  a  less  powerful  cause  of  move- 
ment than  the  other  above  specified  stimuli,  is  ever 
present ;  so  that  it  ultimately  prevails  and  determines 
the  downward  growth  of  the  root. 

The  primary  radicle  emits  secondary  ones  which 
project  sub-horizontally  ;  and  these  were  observed  in 
one  case  to  circumnutate.  Their  tips  are  also  sensitive 
to  contact,  and  they  are  thus  excited  to  bend  away 
from  any  touching  object;  so  that  they  resemble  in 
these  respects,  as  far  as  they  were  observed,  the 
primary  radicles.  If  displaced  they  resume,  as  Sachs 
has  shown,  their  original  sub-horizontal  position ;  and 
this  apparently  is  due  to  diageotropism.  The  secondary 
radicles  emit  tertiary  ones,  but  these,  in  the  case  of 
the  bean,  are  not  affected  by  gravitation ;  consequently 
they  protrude  in  all  directions.  Thus  the  general 


*  Dr.  Karl  Richter,  who  has  in  Wien,'  1879,  p.  149),  states  that 
especially  attended  to  this  subject  apheliotropism  does  not  aid  ra- 
('  K.  Akad.  der  Wissenschalten  dicles  in  penetrating  the  ground. 


CHAP.  XII.  CONCLUDING  REMARKS.  553 

arrangement  of  the  three  orders  of  roots  is  excellently 
adapted  for  searching  the  whole  soil  for  nutriment. 

Sachs  has  shown  that  if  the  tip  of  the  primary 
radicle  is  cut  off  (and  the  tip  will  occasionally  be 
gnawed  off  with  seedlings  in  a  state  of  nature)  one  of 
the  secondary  radicles  grows  perpendicularly  down- 
wards, in  a  manner  which  is  analogous  to  the  upward 
growth  of  a  lateral  shoot  after  the  amputation  of 
the  leading  shoot.  We  have  seen  with  radicles  of  the 
bean  that  if  the  primary  radicle  is  merely  vcompressed 
instead  of  being  cut  off,  so  that  an  excess  of  sap  is 
directed  into  the  secondary  radicles,  their  natural  con- 
dition is  disturbed  aiid  they  grow  downwards.  Other 
analogous  facts  have  been  given.  As  anything  which 
disturbs  the  constitution  is  apt  to  lead  to  reversion, 
that  is,  to  the  resumption  of  a  former  character,  it 
appears  probable  that  when  secondary  radicles  grow 
downwards  or  lateral  shoots  upwards,  they  revert  to 
the  primary  manner  of  growth  proper  to  radicles  and 
shoots. 

With  dicotyledonous  seeds,  after  the  protrusion  of 
the  radicle,  the  hypocotyl  breaks  through  the  seed- 
coats  ;  but  if  the  cotyledons  are  hypogean,  it  is  the 
epicotyl  which  breaks  forth.  These  organs  are  at  first 
invariably  arched,  with  the  upper  part  bent  back 
parallel  to  the  lower ;  and  they  retain  this  form  until 
they  have  risen  above  the  ground.  In  some  cases, 
however,  it  is  the  petioles  of  the  cotyledons  or  of  the 
first  true  leaves  which  break  through  the  seed-coats 
as  well  as  the  ground,  before  any  part  of  the  stem 
protrudes ;  and  then  the  petioles  are  almost  invariably 
arched.  We  have  met  with  only  one  exception,  and  that 
only  a  partial  one,  namely,  with  the  petioles  of  the  two 
first  leaves  of  Acanthus  candelabrum.  With  Delphinium 
nudicaule  the  petioles  of  the  two  cotyledons  are  com- 


554  SUMMARY  AND  CHAP.  XII. 

pletely  confluent,  and  they  break  through  the  ground 
as  an  arch  ;  afterwards  the  petioles  of  the  successively 
formed  early  leaves  are  arched,  and  they  are  thus 
enabled  to  break  through  the  base  of  the  confluent 
petioles  of  the  cotyledons.  In  the  case  of  Megarrhiza, 
it  is  the  plumule  which  breaks  as  an  arch  through  the 
tube  formed  by  the  confluence  of  the  cotyledon- 
petioles.  With  mature  plants,  the  flower-stems  and 
the  leaves  of  some  few  species,  and  the  rachis  of 
several  ferns,  as  they  emerge  separately  from  the 
ground,  are  likewise  arched. 

The  fact  of  so  many  different  organs  in  plants  of 
many  kinds  breaking  through  the  ground  under  the 
form  of  an  arch,  shows  that  this  must  be  in  some 
manner  highly  important  to  them.  According  to 
Haberlandt,  the  tender  growing  apex  is  thus  saved 
from  abrasion,  and  this  is  probably  the  true  explana- 
tion. But  as  both  legs  of  the  arch  grow,  their  power 
of  breaking  through  the  ground  will  be  much  in- 
creased as  long  as  the  tip  remains  within  the  seed- 
coats  and  has  a  point  of  support.  In  the  case  of 
monocotyledons  the  plumule  or  cotyledon  is  rarely 
arched,  as  far  as  we  have  seen ;  but  this  is  the  case 
with  the  leaf-like  cotyledon  of  the  onion ;  and  the 
crown  of  the  arch  is  here  strengthened  by  a  special 
protuberance.  In  the  Gramineae  the  summit  of  the 
straight,  sheath-like  cotyledon  is  developed  into  a 
hard  sharp  crest,  which  evidently  serves  for  breaking 
through  the  earth.  With  dicotyledons  the  arching  of 
the  epicotyl  or  hypocotyl  often  appears  as  if  it  merely 
resulted  from  the  manner  in  which  the  parts  are 
packed  within  the  seed;  but  it  is  doubtful  whether 
this  is  the  whole  of  the  truth  in  any  case,  and  it  cer- 
tainly was  not  so  in  several  cases,  in  which  the  arch- 
ing was  seen  to  commence  after  the  parts  had  wholly 


CHAP.  XII.  CONCLUDING  REMARKS.  555 

escaped  from  the  seed-coats.  As  the  arching  occurred 
in  whatever  position  the  seeds  were  placed,  it  is  no 
doubt  due  to  temporarily  increased  growth  of  the 
nature  of  epinasty  or  hyponasty  along  one  side  of  the 
part. 

As  this  habit  of  the  hypocotyl  to  arch  itself  appears 
to  be  universal,  it  is  probably  of  very  ancient  origin. 
It  is  therefore  not  surprising  that  it  should  be  in- 
herited, at  least  to  some  extent,  by  plants  having 
hypogean  cotyledons,  in  which  the  hypocotyl  is  only 
slightly  developed  and  never  protrudes  above  the 
ground,  and  in  which  the  arching  is  of  course  now 
quite  useless.  This  tendency  explains,  as  we  have 
seen,  the  curvature  of  the  hypocotyl  (and  the  conse- 
quent movement  of  the  radicle)  which  was  first 
observed  by  Sachs,  and  which  we  have  often  had  to 
refer  to  as  Sachs'  curvature. 

The  several  foregoing  arched  organs  are  continually 
circumnutating,  or  endeavouring  to  circumntitate,  even 
before  they  break  through  the  ground.  As  soon  as 
any  part  of  the  arch  protrudes  from  the  seed-coats  it 
is  acted  upon  by  apogeotropism,  and  both  the  legs 
bend  upwards  as  quickly  as  the  surrounding  earth  will 
permit,  until  the  arch  stands  vertically.  By  continued 
growth  it  then  forcibly  breaks  through  the  ground ; 
but  as  it  is  continually  striving  to  circumnutate  this 
will  aid  its  emergence  in  some  slight  degree,  for  we 
know  that  a  circumnutating  hypocotyl  can  push  away 
damp  sand  on  all  sides.  As  soon  as  the  faintest  ray  of 
light  reaches  a  seedling,  heliotropism  will  guide  it 
through  any  crack  in  the  soil,  or  through  an  entangled 
mass  of  overlying  vegetation;  for  apogeotropism  by 
itself  can  direct  the  seedling  only  blindly  upwards. 
Hence  probably  it  is  that  sensitiveness  to  light  resides 
in  the  tip  of  the  cotyledons  of  the  Graminese,  and  in 


556  SUMMARY  AND  CHAF.  XII. 

the  upper  part  of  the  hypocotyls  of  at  least  some 
plants. 

As  the  arch  grows  upwards  the  cotyledons  are 
dragged  out  of  the  ground.  The  seed-coats  are  either 
left  behind  buried,  or  are  retained  for  a  time  still 
enclosing  the  cotyledons.  These  are  afterwards  cast 
off  merely  by  the  swelling  of  the  cotyledons.  But 
with  most  of  the  Cucurbitaceae  there  is  a  curious 
special  contrivance  for  bursting  the  seed-coats  whilst 
beneath  the  ground,  namely,  a  peg  at  the  base  of  the 
hypocotyl,  projecting  at  right  angles,  which  holds  down 
the  lower  half  of  the  seed-coats,  whilst  the  growth 
of  the  arched  part  of  the  hypocotyl  lifts  up  the  upper 
half,  and  thus  splits  them  in  twain.  A  somewhat 
analogous  structure  occurs  in  Mimosa  pudica  and  some 
other  plants.  Before  the  cotyledons  are  fully  ex- 
panded 'and  have  diverged,  the  hypocotyl  generally 
straightens  itself  by  increased  growth  along  the  con- 
cave side,  thus  reversing  the  process  which  caused 
the  arching.  Ultimately  not  a  trace  of  the  former 
curvature  is  left,  except  in  the  case  of  the  leaf-like 
cotyledons  ^of  the  onion. 

The  cotyledons  can  now  assume  the  function  of 
leaves,  and  decompose  carbonic  acid ;  they  also  yield 
up  to  other  parts  of  the  plant  the  nutriment  which 
they  often  contain.  When  they  contain  a  large  stock 
of  nutriment  they  generally  remain  buried  beneath 
the  ground,  owing  to  the  small  development  of  the 
hypocotyl ;  and  thus  they  have  a  better  chance  of 
escaping  destruction  by  animals.  From  unknown 
causes,  nutriment  is  sometimes  stored  in  the  hypocotyl 
or  in  the  radicle,  and  then  one  of  the  cotyledons  or 
both  become  rudimentary,  of  which  several  instances 
have  been  given.  It  is  probable  that  the  extraordi- 
nary manner  of  germination  of  Megarrhiza  Californicay 


CHAP.  XII.  CONCLUDING  REMARKS.  557 

Ipomcea  leptopliylla  and  pandurata,  and  of  Quercus 
virens,  is  connected  with  the  burying  of  the  tuber-like 
roots,  which  at  an  early  age  are  stocked  with  nutri- 
ment ;  for  in  these  plants  it  is  the  petioles  of  the 
cotyledons  which  first  protrude  from  the  seeds,  and 
they  are  then  merely  tipped  with  a  minute  radicle  and 
hypocotyl.  These  petioles  bend  down  geotropically 
like  a  root  and  penetrate  the  ground,  so  that  the  true 
root,  which  afterwards  becomes  greatly  enlarged,  is 
buried  at  some  little  depth  beneath  the  surface.  Gra- 
dations of  structure  are  always  interesting,  and  Asa 
Gray  informs  us  that  with  Ipomcea  Jalappa,  which 
likewise  forms  huge  tubers,  the  hypocotyl  is  still  of 
considerable  length,  and  the  petioles  of  the  cotyledons 
are  only  moderately  elongated.  But  in  addition  to  the 
advantage  gained  by  the  concealment  of  the  nutritious 
matter  stored  within  the  tubers,  the  plumule,  at  least 
in  the  case  of  Megarrhiza,  is  protected  from  the  frosts 
of  winter  by  being  buried. 

With  many  dicotyledonous  seedlings,  as  has  lately 
been  described  by  De  Yries,  the  contraction  of  the 
parenchyma  of  the  upper  part  of  the  radicle  drags  the 
hypocotyl  downwards  into  the  earth ;  sometimes  (it  is 
said)  until  even  the  cotyledons  are  buried.  The  hypo- 
cotyl itself'  of  some  species  contracts  in  a  like  manner. 
It  is  believed  that  this  burying  process  serves  te 
protect  the  seedlings  against  the  frosts  of  winter. 

Our  imaginary  seedling  is  now  mature  as  a  seedling, 
for  its  hypocotyl  is  straight  and  its  cotyledons  are 
fully  expanded.  In  this  state  the  upper  part  of  the 
hypocotyl  and  the  cotyledons  continue  for  some  time 
to  circumnutate,  generally  to  a  wide  extent  relatively 
to  the  size  of  the  parts,  and  at  a  rapid  rate.  But 
seedlings  profit  by  this  power  of  movement  only  when 
it  is  modified,  especially  by  the  action  of  light  and 


55S  SUMMARY   AND  CHAP.  XII. 

gravitation  ;  for  they  are  thus  enabled  to  move  more 
rapidly  and  to  a  greater  exten.t  than  can  most  mature 
plants.  Seedlings  are  subjected  to  a  severe  struggle 
for  life,  and  it  appears  to  be  highly  important  to  them 
that  they  should  adapt  themselves  as  quickly  and  as 
perfectly  as  possible  to  their  conditions.  Hence  also 
it  is  that  they  are  so  extremely  sensitive  to  light  and 
gravitation.  The  cotyledons  of  some  few  species  are 
sensitive  to  a  touch ;  but  it  is  probable  that  this  is 
only  an  indirect  result  of  the  foregoing  kinds  of  sen- 
sitiveness, for  there  is  no  reason  to  believe  that  they 
profit  by  moving  when  touched. 

Our  seedling  now  throws  up  a  stem  bearing  leaves, 
and  often  branches,  all  of  which  whilst  young  are  con- 
tinually circumnutating.  If  we  look,  for  instance,  at  a 
great  acacia  tree,  we  may  feel  assured  that  every  one  of 
the  innumerable  growing  shoots  is  constantly  describ- 
ing small  ellipses  ;  as  is  each  petiole,  sub-petiole,  and 
leaflet.  The  latter,  as  well~as  ordinary  leaves,  gene- 
rally move  up  and  down  in  nearly  the  same  vertical 
plane,  so  that  they  describe  very  narrow  ellipses. 
The  flower-peduncles  are  likewise  continually  circum- 
nutating. If  we  could  look  beneath  the  ground,  and 
our  eyes  had  the  power  of  a  microscope,  we  should  see 
the  tip  of  each  rootlet  endeavouring  to  sweep  small 
ellipses  or  circles,  as  far  as  the  pressure  of  the  sur- 
rounding earth  permitted.  All  this  astonishing  amount 
of  movement  has  been  going  on  year  after  year  since 
the  time  when,  as  a  seedling,  the  tree  first  emerged 
from  the  ground. 

Stems  are  sometimes  developed  into  long  runners  or 
stolons.  These  circumnutate  in  a  conspicuous  manner,  and 
are  thus  aided  in  passing  between  and  over  surrounding 
obstacles.  But  whether  the  circumnutating  movement 
has  been  increased  for  this  special  purpose  is  doubtful. 


CHAP.  XII.  CONCLUDING  EEMARKS.  559 

We  have  now  to  consider  circumnutation  in  a 
modified  form,  as  the  source  of  several  great  classes  of 
movement.  The  modification  may  be  determined  by 
innate  causes,  or  by  external  agencies.  Under  the  first 
head  we  see  leaves  which,  when  first  unfolded,  stand 
in  a  vertical  position,  and  gradually  bend  downwards 
as  they  grow  older.  We  see  flower-peduncles  bending 
down  after  the  flower  has  withered,  and  others  rising 
up  ;  or  again,  stems  with  their  tips  at  first  bowed 
downwards,  so  as  to  be  hooked,  afterwards  straighten- 
ing themselves ;  and  many  other  such  cases.  These 
changes  of  position,  which  are  due  to  epinasty  or 
hyponasty,  occur  at  certain  periods  of  the  life  of  the 
plant,  and  are  independent  of  any  external  agency. 
They  are  effected  not  by  a  continuous  upward  or 
downward  movement,  but  by  a  succession  of  small 
ellipses,  or  by  zigzag  lines, — that  is,  by  a  circum- 
nutating  movement  which  is  preponderant  in  some 
one  direction. 

Again,  climbing  plants  whilst  young  circumnutate 
in  the  ordinary  manner,  but  as  soon  as  the  stem 
has  grown  to  a  certain  height,  which  is  different  for 
different  species,  it  elongates  rapidly,  and  now  the 
amplitude  of  the  circumnutating  movement  is  im- 
mensely increased,  evidently  to  favour  the  stem  catch- 
ing hold  of  a  support.  The  stem  also  circumnutates 
rather  more  equally  to  all  sides  than  in  the  case  of 
non-climbing  plants.  This  is  conspicuously  the  case 
with  those  tendrils  which  consist  of  modified  leaves, 
as  these  sweep  wide  circles ;  whilst  ordinary  leaves 
usually  circumnutate  nearly  in  the  same  vertical  plane. 
Flower-peduncles  when  converted  into  tendrils  have 
their  circumnutating  movement  in  like  manner  greatly 
increased. 

We  now  come  to  our  second  group  of  circumnu- 


560  SUMMARY  AND  CHAP.  XII. 

tating  movements — those  modified  through  external 
agencies.  The  so-called  sleep  or  nyctitropic  move- 
ments of  leaves  are  determined  by  the  daily  alterna- 
tions of  light  and  darkness.  It  is  not  the  darkness 
which  excites  them  to  move,  but  the  difference  in  the 
amount  of  light  which  they  receive  during  the  day 
and  night ;  for  with  several  species,  if  the  leaves  have 
not  been  brightly  illuminated  during  the  day,  they 
do  not  sleep  at  night.  They  inherit,  however,  some 
tendency  to  move  at  the  proper  periods,  indepen- 
dently of  any  change  in  the  amount  of  light.  The 
movements  are  in  some  cases  extraordinarily  complex, 
but  as  a  full  summary  has  been  given  in  the  chapter 
devoted  to  this  subject,  we  will  here  say  but  little  on 
this  head.  Leaves  and  cotyledons  assume  their  noc- 
turnal position  by  two  means,  by  the  aid  of  pulvini  and 
without  such  aid.  In  the  former  case  the  movement 
continues  as  long  as  the  leaf  or  cotyledon  remains  in 
full  health ;  whilst  in  the  latter  case  it  continues  only 
whilst  the  part  is  growing.  Cotyledons  appear  to 
sleep  in  a  larger  proportional  number  of  species  than 
do  leaves.  In  some  species,  the  leaves  sleep  and  not 
the  cotyledons  ;  in  others,  the  cotyledons  and  not  the 
leaves ;  or  both  may  sleep,  and  yet  assume  widely 
different  positions  at  night. 

Although  the  nyctitropic  movements  of  leaves  and 
cotyledons  are  wonderfully  diversified,  and  sometimes 
differ  much  in  the  species  of  the  same  genus,  yet  the 
blade  is  always  placed  in  such  a  position  at  night,  that 
its  upper  surface  is  exposed  as  little  as  possible  to  full 
radiation.  We  cannot  doubt  that  this  is  the  object 
gained  by  these  movements ;  and  it  has  been  proved 
that  leaves  exposed  to  a  clear  sky,  with  their  blades 
compelled  to  remain  horizontal,  suffered  much  more 
from  the  cold  than  others  which  were  allowed  to  assume 


Cu\r.  XII.  CONCLUDING   REMARKS.  561 

their  proper  vertical  position.  Some  curious  facts 
have  been  given  under  this  head,  showing  that  hori- 
zontally extended  leaves  suffered  more  at  night,  when 
the  air,  which  is  not  cooled  by  radiation,  was  prevented 
from  freely  circulating  beneath  their  lower  surfaces  ; 
and  so  it  was,  when  the  leaves  were  allowed  to  go  to 
sleep  on  branches  which  had  been  rendered  motionless. 
In  some  species  the  petioles  rise  up  greatly  at  night, 
and  the  pinnae  close  together.  The  whole  plant  is 
thus  rendered  more  compact,  and  a  much  smaller 
surface  is  exposed  to  radiation. 

That  the  various  nyctitropic  movements  of  leaves 
result  from  modified  circumnutation  has,  we  think, 
been  clearly  shown.  In  the  simplest  cases  a  leaf 
describes  a  single  large  ellipse  during  the  24  h. ;  and 
the  movement  is  so  arranged  that  the  blade  stands 
vertically  during  the  night,  and  reassumes  its  former 
position  on  the  following  morning.  The  course  pursued 
differs  from  ordinary  circumnutation  only  in  its  greater 
amplitude,  and  in  its  greater  rapidity  late  in  the 
evening  and  early  on  the  following  morning.  Unless 
this  movement  is  admitted  to  be  one  of  circumnu- 
tation, such  leaves  do  not  circumnutate  at  all,  and  this 
would  be  a  monstrous  anomaly.  In  other  cases,  leaves 
and  cotyledons  describe  several  vertical  ellipses  during 
the  24  h. ;  and  in  the  evening  one  of  them  is  increased 
greatly  in  amplitude  until  the  blade  stands  vertically 
either  upwards  or  downwards.  In  this  position  it  con- 
tinues to  circumnutate  until  the  following  morning, 
when  it  reassumes  its  former  position.  These  move- 
ments, when  a  pulvinus  is  present,  are  often  compli- 
cated by  the  rotation  of  the  leaf  or  leaflet ;  and  such 
rotation  on  a  small  scale  occurs  during  ordinary  cir- 
cumnutation. The  many  diagrams  showing  the  move- 
ments of  sleeping  and  non-sleeping  leaves  and  coty- 


562  SUMMARY   AND  CHAP.  XII. 

ledons  should  be  compared,  and  it  will  be  seen  that 
they  are  essentially  alike.  Ordinary  circumnutation 
is  converted  into  a  nyctitropic  movement,  firstly  by  an 
increase  in  its  amplitude,  but  not  to  so  great  a  degree 
as  in  the  case  of  climbing  plants,  and  secondly  by  its 
being  rendered  periodic  in  relation  to  the  alterna- 
tions of  day  and  night.  But  there  is  frequently  a 
distinct  trace  of  periodicity  in  the  circumnutating 
movements  of  non-sleeping  leaves  and  cotyledons. 
The  fact  that  nyctitropic  movements  occur  in  species 
distributed  in  many  families  throughout  the  whole 
vascular  series,  is  intelligible,  if  they  result  from  the 
modification  of  the  universally  present  movement  of 
circumnutation  ;  otherwise  the  fact  is  inexplicable. 

In  the  seventh  chapter  we  have  given  the  case  of 
a  Porlieria,  the  leaflets  of  which  remained  closed  all 
day,  as  if  asleep,  when  the  plant  was  kept  dry,  appa- 
rently for  the  sake  of  checking  evaporation.  Some- 
thing of  the  same  kind  occurs  with  certain  Graminese. 
At  the  close  of  this  same  chapter,  a  few  observations 
were  appended  on  what  may  be  called  the  embryology 
of  leaves.  The  leaves  produced  by  young  shoots  on 
cut-down  plants  of  Melilotus  taurica  slept  like  those  of 
a  Trifolium,  whilst  the  leaves  on  the  older  branches 
on  the  same  plants  slept  in  a  very  different  manner, 
proper  to  the  genus ;  and  from  the  reasons  assigned 
we  are  tempted  to  look  at  this  case  as  one  of  reversion 
to  a  former  nyctitropic  habit.  So  again  with  Desmo- 
dium  gyrans,  the  absence  of  small  lateral  leaflets  on 
very  young  plants,  makes  us  suspect  that  the  imme- 
diate progenitor  of  this  species  did  not  possess  lateral 
leaflets,  and  that  their  appearance  in  an  almost  rudi- 
mentary condition  at  a  somewhat  more  advanced  age 
is  the  result  of  reversion  to  a  trifoliate  predecessor. 
However  this  may  be,  the  rapid  circumnutating  or 


CHAP.  XII.  CONCLUDING   REMARKS.  5(33 

gyrating  movements  of  the  little  lateral  leaflets,  seem 
to  be  due  proxirnately  to  the  pulvinus,  or  organ  of 
movement,  not  having  been  reduced  nearly  so  much 
as  the  blade,  during  the  successive  modifications 
through  which  the  species  has  passed. 

We  now  come  to  the  highly  important  class  of 
movements  due  to  the  action  of  a  lateral  light.  When 
sterns,  leaves,  or  other  organs  are  placed,  so  that  one 
side  is  illuminated  more  brightly  than  the  other,  they 
bend  towards  the  light.  This  heliotropic  movement 
manifestly  results  from  the  modification  of  ordinary 
circumnutation ;  and  every  gradation  between  the  two 
movements  could  be  followed.  When  the  light  was 
dim,  and  only  a  very  little  brighter  on  one  side  than 
on  the  other,  the  movement  consisted  of  a  succession 
of  ellipses,  directed  towards  the  light,  each  of  which 
approached  nearer  to  its  source  than  the  previous  one. 
When  the  difference  in  the  light  on  the  two  sides 
was  somewhat  greater,  the  ellipses  were  drawn  out 
into  a  strongly-marked  zigzag  line,  and  when  much 
greater  the  course  became  rectilinear.  We  have 
reason  to  believe  that  changes  in  the  turgescence  of 
the  cells  is  the  proximate  cause  of  the  movement 
of  circumnutation ;  and  it  appears  that  when  a  plant 
is  unequally  illuminated  on  the  two  sides,  the  always 
changing  turgescence  is  augmented  along  one  side, 
and  is  weakened  or  quite  arrested  along  the  other 
sides.  Increased  turgescence  is  commonly  followed  by 
increased  growth,  so  that  a  plant  which  has  bent  itself 
towards  the  light  during  the  day  would  be  fixed  in  this 
position  were  it  not  for  apogeotropism  acting  during 
the  night.  But  parts  provided  with  pulvini  bend,  as 
Pfeffer  has  shown,  towards  the  light ;  and  here  growth 
does  not  come  into  play  any  more  than  in  the  ordinary 
circumnutating  movements  of  pulvini. 


564  SUMMARY    AND  CHAP.  XII. 

Heliotropisin  prevails  widely  throughout  the  vege- 
table kingdom,  but  whenever,  from  the  changed  habits 
of  life  of  any  plant,  such  movements  become  injurious 
or  useless,  the  tendency  is  easily  eliminated,  as  we  see 
with  climbing  and  insectivorous  plants. 

Apheliotropic  movements  are  comparatively  rare  in 
a  well-marked  degree,  excepting  with  sub-aerial  roots. 
In  the  two  cases  investigated  by  us,  the  movement 
certainly  consisted  of  modified  circumnutation. 

The  position  which  leaves  and  cotyledons  occupy 
during  the  day,  namely,  more  or  less  transversely  to 
the  direction  of  the  light,  is  due,  accoroUng  to  Frank, 
to  what  we  call  diaheliotropism.  As  all  leaves  and 
cotyledons  are  continually  circumnutating,  there  can 
hardly  be  a  doubt  that  diaheliotropism  results  from 
modified  circumnutation.  From  the  fact  of  leaves  and 
cotyledons  frequently  rising  a  little  in  the  evening,  it 
appears  as  if  diaheliotropism  had  to  conquer  during 
the  middle  of  the  day  a  widely  prevalent  tendency  to 
apogeotropism. 

Lastly,  the  leaflets  and  cotyledons  of  some  plants 
are  known  to  be  injured  by  too  much  light;  and  when 
the  sun  shines  brightly  on  them,  they  move  upwards 
or  downwards,  or  twist  laterally,  so  that  they  direct 
their  edges  towards  the  light,  and  thus  they  escape 
being  injured.  These  paraheliotropic  movements  cer- 
tainly consisted  in  one  case  of  modified  circumnuta- 
.tion ;  and  so  it  probably  is  in  all  cases,  for  the  leaves 
of  all  the  species  described  circumnutate  in  a  con- 
spicuous manner.  This  movement  has  hitherto  been 
observed  only  with  leaflets  provided  with  pulvini,  in 
which  the  increased  turgescence  on  opposite  sides  is 
not  followed  by  growth ;  and  we  can  understand  why 
this  should  be  so,  as  the  movement  is  required  only 
for  a  temporary  purpose.  It  would  manifestly  be  dis- 


CHAP.  XII  CONCLUDING    REMARKS.  565 

advantageous  for  the  leaf  to  be  fixed  by  growth  in  its 
inclined  position.  For  it  has  to  assume  its  former 
horizontal  position,  as  soon  as  possible  after  the  sun 
has  ceased  shining  too  brightly  on  it. 

The  extreme  sensitiveness  of  certain  seedlings  to 
light,  as  shown  in  our  ninth  chapter,  is  highly  remark- 
able. The  cotyledons  of  Phalaris  became  curved 
towards  a  distant  lamp,  which  emitted  so  little  light, 
that  a  pencil  held  vertically  close  to  the  plants,  did 
not  cast  any  shadow  which  the  eye  oould  perceive 
on  a  white  card.  These  cotyledons,  therefore,  were 
affected  by  a  difference  in  he  amount  of  light  on  their 
two  sides,  which  the  eye  could  not  distinguish.  The 
degree  of  their  curvature  within  a  given  time  towards 
a  lateral  light  did  not  correspond  at  all  strictly  with 
the  amount  of  light  which  they  received ;  the  light 
not  being  at  any  time  in  excess.  They  continued  for 
nearly  half  an  hour  to  bend  towards  a  lateral  light, 
after  it  had  been  extinguished.  They  bend  with 
remarkable  precision  towards  it,  and  this  depends  on 
the  illumination  of  one  whole  side,  or  on  the  obscura- 
tion of  the  whole  opposite  side.  The  difference  in  the 
amount  of  light  which  plants  at  any  time  receive  in 
comparison  with  what  they  have  shortly  before  re- 
ceived, seems  in  all  cases  to  be  the  chief  exciting  cause 
of  those  movements  which  •  are  influenced  by  light. 
Thus  seedlings  brought  out  of  darkness  bend  towards 
a  dim  lateral  light,  sooner  than  others  which  had  pre-^ 
viously  been  exposed  to  daylight.  We  have  seen 
several  analogous  cases  with  the  nyctitropic  move- 
ments of  leaves.  A  striking  instance  was  observed  in 
the  case  of  the  periodic  movements  of  the  cotyledons 
of  a  Cassia;  in  the  morning  a  pot  was  placed  in  an 
obscure  part  of  a  room,  and  all  the  cotyledons  rose  up 
closed ;  another  pot  had  stood  in  the  sunlight,  and 


566  SUMMARY    AND  CHAP.  X1T. 

the  cotyledons  of  course  remained  expanded ;  both 
pots  were  now  placed  close  together  in  the  middle  of 
the  room,  and  the  cotyledons  which  had  been  exposed 
to  the  sun,  immediately  began  to  close,  while  the 
others  opened ;  so  that  the  cotyledons  in  the  two  pots 
moved  in  exactly  opposite  directions  whilst  exposed 
to  the  same  degree  of  light. 

We  found  that  if  seedlings,  kept  in  a  dark  place, 
were  laterally  illuminated  by  a  small  wax  taper  for 
only  two  or  three  minutes  at  intervals  of  about  three- 
quarters  of  an  hour,  they  all  became  bowed  to  the 
point  where  the  taper  had  been  held.  We  felt  much 
surprised  at  this  fact,  and  until  we  had  read  Wiesner's 
observations,  we  attributed  it  to  the  after-effects  of 
the  light;  but  he  has  shown  that  the  same  degree 
of  curvature  in  a  plant  may  be  induced  in  the 
course  of  an  hour  by  several  interrupted  illumina- 
tions lasting  altogether  for  20  m.,  as  by  a  continuous 
illumination  of  60  m.  We  believe  that  this  case, 
as  well  as  our  own,  may  be  explained  by  the  ex- 
citement from  light  being  due  not  so  much  to  its 
actual  amount,  as  to  the  difference  in  amount  from 
that  previously  received ;  and  in  our  case  there  were 
repeated  alternations  from  complete  darkness  to  light. 
In  this,  and  in  several  of  the  above  specified  respects, 
light  seems  to  act  on  the'  tissues  of  plants,  almost  in 
the  same  manner  as  it  does  on  the  nervous  system 
of  animals. 

There  is  a  much  more  striking  analogy  of  the  same 
kind,  in  the  sensitiveness  to  light  being  localised  in 
the  tips  of  the  cotyledons  of  Phalaris  and  A  vena,  and 
in  the  upper  part  of  the  hypocotyls  of  Brassica  and 
Beta  ;  and  in  the  transmission  of  some  influence  from 
these  upper  to  the  lower  parts,  causing  the  latter  to 
bend  towards  the  light.  This  influence  is  also  trans- 


CHAP.  XII.  CONCLUDING   REMARKS.  567 

mitted  beneath  the  soil  to  a  depth  where  no  light- 
enters.  It  follows  from  this  localisation,  that  the 
lower  parts  of  the  cotyledons  of  Phalaris,  &c.,  which 
normally  become  more  bent  towards  a  lateral  light 
than  the  upper  parts,  may  be  brightly  illuminated 
during  many  hours,  and  will  not  bend  in  the  least,  if 
all  light  be  excluded  from  the  tip.  It  is  an  interest- 
ing experiment  to  place  caps  over  the  tips  of  the 
cotyledons  of  Phalaris,  and  to  allow  a  very  little  light 
to  enter  through  minute  orifices  on  one  side  of  the 
caps,  for  the  lower  part  of  the  cotyledons  will  then 
bend  to  this  side,  and  not  to  the  side  which  has  been 
brightly  illuminated  during  the  whole  time.  In  the 
case  of  the  radicles  of  Sinapis  alba,  sensitiveness  to 
light  also  resides  in  the  tip,  which,  when  laterally 
illuminated,  causes  the  adjoining  part  of  the  root  to 
bend  apheliotropically. 

Gravitation  excites  plants  to  bend  away  from  the 
centre  of  the  earth,  or  towards  it,  or  to  place  them- 
selves in  a  transverse  position  with  respect  to  it. 
Although  it  is  impossible  to  modify  in  any  direct 
manner  the  attraction  of  gravity,  yet  its  influence 
could  be  moderated  indirectly,  in  the  several  ways 
described  in  the  tenth  chapter ;  and  under  such 
circumstances  the  same  kind  of  evidence  as  that  given 
in  the  chapter  on  Heliotropism,  showred  in  the  plainest 
manner  that  apogeotropic  and  geotropic,  and  probably 
diageotropic  movements,  are  all  modified  forms  of 
circumnutation. 

Different  parts  of  the  same  plant  and  different 
species  are  affected  by  gravitation  in  widely  different 
degrees  and  manners.  Some  plants  and  organs  exhibit 
hardly  a  trace  of  its  action.  Young  seedlings  which, 
as  we  know,  circumnutate  rapidly,  are  eminently  sensi- 
tive ;  and  we  have  seen  the  hypocotyl  of  Beta  bending 
25 


568  SUMMARY   AND  CHAP.  XII. 

upwards  through  109°  in  3  h.  8  m.  The  after-effects 
of  apogeotropisni  last  for  above  half  an  hour ;  and 
horizontally-laid  hypocotyls  are  sometimes  thus  car- 
ried temporarily  beyond  an  upright  position.  The 
benefits  derived  from  geotropism,  apogeotropisni,  and 
diageotropism,  are  generally  so  manifest  that  they 
need  not  be  specified.  With  the  flower-peduncles  of 
Oxalis,  epinasty  causes  them  to  bend  down,  so  that 
the  ripening  pods  may  be  protected  by  the  calyx 
from  the  rain.  Afterwards  they  are  carried  upwards 
by  apogeotropisni  in  combination  with  hyponasty,  and 
are  thus  enabled  to  scatter  their  seeds  over  a  wider 
space.  The  capsules  and  flower-heads  of  some  plants 
are  bowed  downwards  through  geotropism,  and  they 
then  bury  themselves  in  the  earth  for  the  protection 
and  slow  maturation  of  the  seeds.  This  burying 
process  is  much  facilitated  by  the  rocking  movement 
due  to  circumnutation. 

In  the  case  of  the  radicles  of  several,  probably  of  all 
seedling  plants,  sensitiveness  to  gravitation  is  confined 
to  the  tip,  which  transmits  an  influence  to  the  adjoining 
upper  part,  causing  it  to  bend  towards  the  centre  of 
the  earth.  That  there  is  transmission  of  this  kind  was 
proved  in  an  interesting  manner  when  horizontally 
extended  radicles  of  the  bean  were  exposed  to  the 
attraction  of  gravity  for  1  or  1J-  h.,  and  their  tips  were 
then  amputated.  Within  this  time  no  trace  of  curva- 
ture was  exhibited,  and  the  radicles  were  now  placed 
pointing  vertically  downwards ;  but  an  influence  had 
already  been  transmitted  from  the  tip  to  the  adjoining 
part,  for  it  soon  became  bent  to  one  side,  in  the  same 
manner  as  would  have  occurred  had  the*  radicle 
remained  horizontal  and  been  still  acted  on  by  geo- 
tropism. Kadicles  thus  treated  continued  to  grow  out 
horizontally  for  two  or  three  days,  until  a  new  tip  was 


CHAP.  XII.  CONCLUDING  REMAKES.  569 

reformed ;  and  this  was  then  acted  on  by  geotropism, 
and  the  radicle  became  curved  perpendicularly  down- 
wards. 

It  has  now  been  shown  that  the  following  important 
classes  of  movement  all  arise  from  modified  circum- 
nutation,  which  is  omnipresent  whilst  growth  lasts, 
and  after  growth  has  ceased,  whenever  pulvini  are 
present.  These  classes  of  movement  consist  of  those 
due  to  epinasty  and  hyponasty, — those  proper  to 
climbing  plants,  commonly  called  revolving  nutation, 
— the  nyctitropic  or  sleep  movements  of  leaves  and 
cotyledons, — and  the  two  immense  classes  of  move- 
ment excited  by  light  and  gravitation.  When  we 
speak  of  modified  circumnutation  we  mean  that  light, 
or  the  alternations  of  light  and  darkness,  gravitation, 
slight  pressure  or  other  irritants,  and  certain  innate 
or  constitutional  states  of  the  plant,  do  not  directly 
cause  the  movement ;  they  merely  lead  to  a  tempo- 
rary increase  or  diminution  of  those  spontaneous 
changes  in  the  turgescence  of  the  cells  which  are 
already  in  progress.  In  what  manner,  light,  gravita- 
tion, &c.,  act  on  the  cells  is  not  known ;  and  we 
will  here  only  remark  that,  if  any  stimulus  affected 
the  cells  in  such  a  manner  as  to  cause  some  slight 
tendency  in  the  affected  part  to  bend  in  a  beneficial 
manner,  this  tendency  might  easily  be  increased 
through  the  preservation  of  the  more  sensitive  indi- 
viduals. But  if  such  bending  were  injurious,  the 
tendency  would  be  eliminated  unless  it  was  over- 
powering! y  strong;  for  we  know  how  commonly  all 
characters  in  all  organisms  vary.  Nor  can  we  see  any 
reason  to  doubt,  that  after  the  complete  elimination  of 
a  tendency  to  bend  in  some  one  direction  under  a 
certain  stimulus,  the  power  to  bend  in  a  directly 


570  SUMMAKY   AND  CHAP.  XIT. 

opposite  direction  might  gradually  be  acquired  through 
natural  selection.* 

Although  so  many  movements  have  arisen  through 
modified  circumnutation,  there  are  others  which 
appear  to  have  had  a  quite  independent  origin;  but 
they  do  not  form  such  large  and  important  classes. 
When  a  leaf  of  a  Mimosa  is  touched  it  suddenly 
assumes  the  same  position  as  when  asleep,  but  Briicke 
has  shown  that  this  movement  results  from  a  different 
state  of  turgescence  in  the  cells  from  that  which 
occurs  during  sleep  ;  and  as  sleep-movements  are  cer- 
tainly due  to  modified  circumnutation,  those  from  a 
touch  can  hardly  be  thus  due.  The  back  of  a  leaf  of 
Drosera  rotundifolia  was  cemented  to  the  summit  of 
a  stick  driven  into  the  ground,  so  that  it  could  not 
move  in  the  least,  and  a  tentacle  was  observed  during 
many  hours  under  the  microscope;  but  it  exhibited 
110  circumnutating  movement,  yet  after  being  mo- 
mentarily touched  with  a  bit  of  raw  meat,  its  basal 
part  began  to  curve  in  23  seconds.  This  curving 
movement  therefore  could  not  have  resulted  from 
modified  circumnutation.  But  when  a  small  object, 
such  as  a  fragment  of  a  bristle,  was  placed  on  one  side 
of  the  tip  of  a  radicle,  which  we  know  is  continually 
circumnutating,  the  induced  curvature  was  so  similar 
to  the  movement  caused  by  geotropism,  that  we  can 
hardly  doubt  that  it  is  due  to  modified  circumnu- 
tation. A  flower  of  a  Mahonia  was  cemented  to  a 
stick,  and  the  stamens  exhibited  no  signs  of  circum- 
nutation under  the  microscope,  yet  when  they  were 
lightly  touched  they  suddenly  moved  towards  the  pistil. 
Lastly,  the  curling  of  the  extremity  of  a  tendril  when 


*  See  the  remaiks  in  Frank's  91,  &c  ),  on  natural  selection  in 
*Die  wagerechte  Idchtung  von  connection  with  geotropisni,  helio- 
Pnauzentheilen '  (1870,  pp.  90,  tropism,  &c. 


CHAP.  XII.  CONCLUDING  REMARKS.  571 

touched  seems  to  be  independent  of  its  revolving  or 
circumnutating  movement.  This  is  best  shown  by  the 
part  which  is  the  most  sensitive  to  contact,  circum- 
nutating much  less  than  the  lower  parts,  or  apparently 
not  at  all.* 

Although  in  these  cases  we  have  no  reason  to 
believe  that  the  movement  depends  on  modified  cir- 
cumnutation,  as  with  the  several  classes  of  movement 
described  in  this  volume,  yet  the  difference  between 
the  two  sets  of  cases  may  not  be  so  great  as  it  at 
first  appears.  In  the  one  set,  an  irritant  causes  an 
increase  or  diminution  in  the  turgescence  of  the  cells, 
which  are  already  in  a  state  of  change  ;  whilst  in  the 
other  set,  the  irritant  first  starts  a  similar  change  in 
their  state  of  turgescence.  Why  a  touch,  slight 
pressure  or  any  other  irritant,  such  as  electricity,  heat, 
or  the  absorption  of  animal  matter,  should  modify  the 
turgescence  of  the  affected  cells  in  such  a  manner  as  to 
cause  movement,  we  do  not  know.  But  a  touch  acts  in 
this  manner  so  often,  and  on  such  widely  distinct  plants, 
that  the  tendency  seems  to  be  a  very  general  one ;  and 
if  beneficial,  it  might  be  increased  to  any  extent.  In 
other  cases,  a  touch  produces  a  very  different  effect, 
as  with  Nitella,  in  which  the  protoplasm  may  be  seen 
to  recede  from  the  walls  of  the  cell;  in  Lactuca,  in 
which  a  milky  fluid  exudes;  and.  in  the  tendrils  of 
certain  Vitaceae,  Cucurbitaceae,  and  Bignoniacese,  in 
which  slight  pressure  causes  a  cellular  outgrowth. 

Finally,  it  is  impossible  not  to  be  struck  with  the 
resemblance  between  the  foregoing  movements  of 
plants  and  many  of  the  actions  performed  uncon- 
sciously by  the  lower  animals.f  With  plants  an 

*  For    the    evidence    on  this       pp.  173,  174. 

head,  see   tl}e   '  Movements   and  f  Sachs  remarks  to  nearly  the 

Habits  of  Climbing  Plants,' 1875,       same  effect :  "  Dass  sich  die  le- 


572  SUMMARY   AND  CHAP.  XII, 

astonishingly  small  stimulus  suffices ;  and  even  with 
allied  plants  one  may  be  highly  sensitive  to  the 
slightest  continued  pressure,  and  another  highly  sensi- 
tive to  a  slight  momentary  touch.  The  habit  of  moving 
at  certain  periods  is  inherited  both  by  plants  and 
animals ;  and  several  other  points  of  similitude  have 
been  specified.  But  the  most  striking  resemblance  is 
the  localisation  of  their  sensitiveness,  and  the  transmis- 
sion of  an  influence  from  the  excited  part  to  another 
which  consequently  moves.  Yet  plants  do  not  of  course 
possess  nerves  or  a  central  nervous  system;  and  we 
may  infer  that  with  animals  such  structures  serve  only 
for  the  more  perfect  transmission  of  impressions,  and 
for  the  more  complete  intercommunication  of  the 
several  parts. 

We  believe  that  there  is  no  structure  in  plants  more 
wonderful,  as  far  as  its  functions  are  concerned,  than 
the  tip  of  the  radicle.  If  the  tip  be  lightly  pressed 
or  burnt  or  cut,  it  transmits  an  influence  to  the  upper 
adjoining  part,  causing  it  to  bend  away  from  the 
affected  side;  and,  what  is  more  surprising,  the  tip 
can  distinguish  between  a  slightly  harder  and  softer 
object,  by  which  it  is  simultaneously  pressed  on  oppo- 
site sides.  If,  however,  the  radicle  is  pressed  by  a 
similar  object  a  'little  above  the  tip,  the  pressed  part 
does  not  transmit  any  influence  to  the  more  distant 
parts,  but  bends  abruptly  towards  the  object.  If  the 
tip  perceives  the  air  to  be  moister  on  one  side  than 
on  the  other,  it  likewise  transmits  an  influence  to  the 
upper  adjoining  part,  which  bends  towards  the  source 
of  moisture.  When  the  tip  is  excited  by  light  (though 


bencle    Pflanzensubstanz    derart  lich,wiedie  verscliieclenen  Sinnes- 

inneilich   differenzirt,    dass    ein-  nerven    des    Thiere'    ('Arbeiten 

zelne     Theile     mil    specifischen  des  Bot.  Inst.  in  Wiirzburg,'  Bd. 

Energien  ausgeriistet    sind,  ahn-  ii.  1879,  p.  282). 


CHAP.  XII.  CONCLUDING  REMARKS.  573 

in  the  case  of  radicles  this  was  ascertained  in  only  a 
single  instance)  the  adjoining  part  bends  from  the 
light ;  but  when  excited  by  gravitation  the  same  part 
bends  towards  the  centre  of  gravity.  In  almost  every 
case  we  can  clearly  perceive  the  final  purpose  or  advan- 
tage of  the  several  movements.  Two,  or  perhaps  more, 
of  the  exciting  causes  often  act  simultaneously  on  the 
tip,  and  one  conquers  the  other,  no  doubt  in  accord- 
ance with  its  importance  for  the  life  of  the  plant. 
The  course  pursued  by  the  radicle  in  penetrating  the 
ground  must  be  determined  by  the  tip ;  hence  it 
has  acquired  such  diverse  kinds  of  sensitiveness.  It 
is  hardly,  an  exaggeration  to  say  that  the  tip  of  the 
radicle  thus  endowed,  and  having  the  power  of 
directing  the  movements  of  the  adjoining  parts,  acts 
like  the  brain  of  one  of  the  lower  animals ;  the  brain 
being  seated  within  the  anterior  end  of  the  body, 
receiving  impressions  from  the  sense-organs,  and 
directing  the  several  movements. 


INDEX. 


ABIES. 


Abies  communis,  effect  of  killing  or 
injuring  the  leading  shoot.  187 

pectinaia,  effect  of  killing  or 

f  injuring  the  leading  shoot,  187 

,  affected  by  2Ecidium  elatinum, 

188 

Abrnnia  umbellatn,  its  single,  deve- 
loped cotyledon,  78 

,  rudimentary  cotyledon,  95 

,  rupture  of  the  seed  coats,  105 

AbutfJon  Darwinii,  sleep  of  leaves 
and  not  of  cotyledons.  314 

.  nocturnal  movement  of  leaves, 

323 

Acacia  Farnesiana,  state  of  plant 
when  awake  and  asleep,  381,  382 

,  appearance  at  night,  395 

,  nyctitropic  movements  of 

pinnre,  402 

,  the  axes  of  the  ellipses,  404 

lopkantka,  character  of  first 

leaf,  415 

retinoides,   circnmnufation   of 

young  phyllode,  236 

Acanlhosicyos  horrida,  nocturnal 
movement  of  cotyledon  30i 

Acanthus  canddalrrum,  inequality  in 
the  two  first  leaves,  79 

,  petioles  not  arched,  553 

latifolius,  variability  in  first 

leaves.  79 

mollis,    seedling,    manner   of 

breaking    through    the    ground, 
78,  79 

,  circumnutation  of  young  leaf, 

249,  269 

gpinozua,  79 

,  movement  of  It  avos,  249 


AMPHICAKPCEA. 

Adenanthera    pavonia,   nyctitropic 

movements  of  leaflets,  374 
££ddi#m   elatinum,    effect   on   the 

lateral  branches  of  the  silver  n'r, 

188 
JEsculus  hipi  ocastanum,  movements 

of  radicle,  28,  2!) 
,  sensitiveness  of  apex  of  radicle, 

172-174 

Albfzzia  lophanthi.,  nycf  itropic  move- 
ments of  leaflets,  383 

— ,  of  pinnae,  402 
Aliiam   cepa,  conical   protuberance 

on  arched  cotyledon,  59 
•,  circumnutation  of  basal  half 

of  arc-he- 1  cotyledon,  60 

— ,   mode    of    breaking  through 

ground,  87 

,  straightening  process.  101 

porrum,  movements  of  flower- 
stems,  22fJ 
Aloptcnriis  pratensis,  joints  affected 

by  apogootropism,  503 
Aloysia  ritriodora,   circumnutation 

ot'btem.  210 

Amaranthus.  sle^p  of  leaves  387 
caudatus,  noctural  movement 

of  cotyledons,  307 
Amorpha  fruticosa.  sleep  of  leaflets, 

354 
Ampelopsis  trieuspidata,  hyponastic 

movement   of  hooked  tips,  272- 

275 

Ampliicarpcea  monoira,  circumnuta- 
tion and  nyctitropic  movements 

of  leaves,  3(55 
,  effect  of  sunshine  on  leaflets, 

445 
,    geotropic      movements     of, 

520 


INDEX. 


575 


ANODA. 

Anoda  Wrigktii,  sleep  of  cotyledons, 
3l)2,  312 

,  of  leaves,  321 

• ,  downward  movement  of  coty- 
ledons, 444 

Aplu liotropism,  or  negative  helio- 
tropi^m,  5,  419,  432 

Apios  graveolens,  heliotropic  move- 
ments of  hypocolyl,  4^2-424 

hiberosa,  vertical  sinking  of 

leaflets  at  night,  368 

Apium  graceolenn,  sleep  of  cotyle- 
dons, 305 

,  petroicllnum,  sleep  of  cotyle- 
dons, 30 1 

Apogeotropic  movements  effected  by 
joints  or  pulvini,  502 

Apogeotropi-mi,  5,  494 ;  retarded  by 
hehotropism,  501  ;  concluding  re- 
marks on.  507 

Aradii*  hypognea,  circumnutation  of 
gynophure,  225 

,  effects  of  radiation  on  leavi  s, 

289,  29, > 

,  movements  of  leaves,  357 

,  rate  of  movement,  404 

,  circumnutation  of  vertically 

dependent  young  gynophores,  519 

,  downward  movement  of  the 

same',  51!) 

Arching  of  various  organs,  impor- 
tance of,  to  seedling  plants,  87, 
88  :  emergence  of  hypocotyls  or 
epicotyls  in  the  form  of  an,  553 

Asjiaragus  officinalis,  circumnuta- 
tion of  plumules,  60-G2. 

,  effect  of  lateral  light,  484 

Asplenium  trichomanes,  movement 
in  the  fruiting  fronds,  257,  n. 

Astragalus  uliginosus,  movement  of 
leaflets,  355 

Avena  sat/va,  movement  of  cotyle- 
dons, (55,  66. 

,  sensitiveness  of  tip  of  radicle 

to  moist  air,  183 

,  heliotropic  movement  and  cir- 

cumnutation  of  cotyledon,  421,422 

,  sensitiveness  of  cotyledon  to  a 

lateral  light,  477 

,  young  sheath-like  cotyledons 

strongly  apogeotropic,  499 


Avena  saliva,  movements  of  oldish 

cotyledons,  4!)9,  500 
Averrlwa  liilimbi,  leaf  asleep,  330 
,     angular     movements    when 

going  t;>  fclt.ep,  331-hd5 
,    leaflets   exposed    to    bright 

sunshine,  417 
Azalea  Indica,   circumnutation  of 

stem,  208 


B. 


Bary,  de,  on  the  effect  of  the  JEci- 
dium  on  the  silver  n'r,  188 

Batiilin,  Piof,  on  the  nvctitropic 
movements  of  leaves,  2^3  ;  on  the 
sh  ep  of  leaves  of'  S>da  nftpcea, 
322  ;  on  Poh/gonum  aciculare, 
387  ;  on  the  t fleet  of  sunshine  ou 
leaflets  of  Oxalis  arefoselln,  447 

Bauhinia,  nyctitiopic  movements, 
373 

,movementsof  petioles  of  young 

keoiilings,  401 

,  appearance  of  young  plants 

at  night,  402 

Beta  vulgaris,  circumnutation  of 
hypocotyl  of  seedlings,  52 

,  movements  of  cotyledon?,  52, 

53 

,  effect  of  light,  124 

,  nocturnal  movement  of  coty- 
ledons, S07 

,  heliolropic  movements  of, 

420 

,  transmitted  tffect  of  light  on 

hypocotyl,  482 

-.  apogeotiopic  movement  of 

hypocotyl,  49tj 

Bignonia  capreolata,  apheliotropic 
movement  of  tendrils,  432,  45u 

Bouehe  on  Melaleuca  ericcefolia, 
383 

Brassica  napus,  circumnutation  of 
flower-stems  226 

Brassica  olvracea,  circumnutation 
of  iteedling,  10 

,  of  radicle,  11 

,  geotropic  movement  of  radicle, 

11 


576 


INDEX. 


BRASSICA. 

Brassica    oleracea,    movement    of 

buried  and  arched  liypocotyl,  13, 

14,  15 
,    conjoint    circumnutalion    of 

hypocotyl  and  cotyledons,  16,  17, 

18 

,  of  hypoeotyl  in  dnrkness,  19 

,  of  a 'cotyledon  with  hypocotyl 

teemed  to  a  stick,  ID,  20 

,  rate  of  movement,  20 

,  ellipses   described    by    hypo- 

cotyls  when  erect,  105 

,  movements  of  cotyledons,  115 

, of  stem,  202 

, of  leaves  at  nighf,  229, 

230 

,  sleep  of  cotyledons,  301 

,  circumnutatioii   of  hypoeotyl 

of  seedling  plant,  425' 
,   heliotropic    movement     and 

circumuutation    of     hypocotyls, 

426 

,  effect  of  lateral  light  on  hypo- 
cotyls, 479-482 
,   apogeotropic    movem  nt    of 

hypocotyls,  500,  501 
Bms»ic.a  rapa,  movements  of  leaves, 

230 
Bronuniart,   A.,   on    the    si  ep    of 

Strephium  floribundnm,  391 
Bruce,  Dr.,  on  the  bleep  of  leaves  in 

Arerrhoa,  330 
Brijophyllum  (vel  Calanchoe)  calyci- 

num,  movement  of  leaves,  237 


C. 


Camellia  Japom'ca,  circnnriutation 
of  leaf,  231,  232 

Candolle,  A.  de,  on  Trapa  natans, 
95  ;  on  sensitiveness  of  coty- 
ledons. 127 

Canna  Warscewic:ii,  circumnuta- 
tion  of  plumules,  58,  59 

,  of  leaf,  254 

Cannabis  saliva,  movements  of 
leaves,  250 

,  nocturnal  movements  of  coty- 

1  dons,  307 


CASSIA. 

Cannabis  sativa,  sinking  of  the  young 
leaves  at  night,  444 

Outsit,    nyctitropic     movement    of 
leaves.  3»>9 

Cassia  Barclay  ana,  nocturnal  move- 
ment of  leaves,  372 
,  slight  movementof  leaflets,  40 1 

calliantha,  uninjured    by  ex- 
posure at  ni^ht,  289.  n. 

,    nyctitropic     movement      of 

Laves,  371 
,  circumnutatiug  movement  of 

leaves,  372 

corymbosa,   cotyledons  sensi- 
tive to  contact,  126 

,     nyctitropic     movement     of 


Laves,  369 

florihnndq,  use  of  sleep  move- 
ments, 289 

,  effect  of  radiation  on  the 

leaves  ac  night,  2':)4 

,  circuMiiiutatiag  and  nycti- 
tropic movement  of  a  terminal 
leaflet,  372,  373 

,  movements  of  young  and  older 

leaves,  400 

florida.  cotyledon's  sensitive  to 

contact.  126 

,  sleep  of  cotyledons,  308 

glauca,  cotyledons  sensitive  to 

contact-,  126 

,  sleep  of  cotyledons,  308 

Ixvigata,  effect  of  radiation 

on  leaves,  289,  n. 

mimosoides,  movement  of  coty- 
ledons. 116 

,  sensitiveness  of,  126 

,  sleep  of,  308 

,  nyctitropic  movement  of 

leaves,  372 

,  effect  of  bright  sunshine  on 

cotyledons,  446 

nenlecta,  movements  of,  117 

,  effect  of  light,  124 

,  sensitiveness  of  cotyledons- 

126 

nodosa,   non-sensitive   cotyle- 
dons. 126 

,  do  not  rise  at  nuht,  308 

pubevcens,  non-sensitive  coty 

ledona,  126 


INDEX. 


577 


CASSIA. 

Cassia  pubescens,  uninjured  by  ex 

posure  at  night.  293 

,  sleep  of  cotyledons,  308 

,     nyctitropic     movement     of 

leaves,  371 
,    circumnutatiug     movement 

of  leaves,  372 
,     nyctitropic     movement     of 

petioles,  400 
,  diameter  of  plant  at  night, 

402 
sp.  (?)  movement  of  cotyledons, 

116 

fora,  circumnutation  of  coty- 
ledons  and    hypocotyls,   34,   35, 
109,  308 

,  effect  of  light,  124,  125 

• ,     sensitiveness     to     contact, 

125 
,    heliotropic    movement    and 

circumnutation     of      hypocotyl, 

431 
,  hypocotyl  of  seedling  slightly 

heliotropic,  454 
,  apogeotropic  movement  of  old 

hypocotyl,  497 
,   movement    of   hypocotyl   of 

young  seedling,  510 
Caustic  (nitrate  of  silver),  effect  of, 

on  radicle  of  bean,  150,  156;  on 

the  common  pea,  1 60. 
Cells,   table    of    the    measurement 

of,    in    the    pulvini    of    Oxalis 

corniculata,    120  ;     changes    in, 

547 

Centrosema.  3o5 

Ceratophyllum     demersum,     move- 
ments of  fetem,  211 
Cert-us  Landbeckii,  its  rudimentary 

cot\  ledons,  97 

speciossimus,   circumnutation 

of  stem,  206,  207 

Cerinthe  major,  circumnutation  of 
hypocotyl,  49 

,  of  cotyledons,  49 

,  ellipses  described  by  hypo- 
cotyls when  erect,  107 

effect  of  darkness,  124 

Chatin,  M.,  on  Pinus  Nordman- 
niana,  389 

Chenopodium      album,     sleep      of 


CRINTJM. 

It  aves,  but  not  of  cotyledons,  314, 
319 

Chenopodium  album,  movement  of 
leaves,  387 

Chlorophyll  injured  by  bright  li^ht, 
446 

Ciesielski,  on  the  sensitiveness  of 
the  tip  of  the  radicles,  4,  523 

Circumnutation,  meaning  explained. 
1 ;  modified,  263-279  ;  and  helio- 
trjpism,  relation  between,  435 ; 
of  paramount  importance  to  every 
plant,  547 

Oissus  discolor,  circumnutation  of 
leaf,  233 

Citrus  aurantium,  circumnutation 
of  eptcotyl,  28 

,  unequal  cotyledons,  95 

Clianthus  Dampieri,  nocturnal 
movement  of  leaves,  297 

Coboea  scaiidens,  circumnutation  of, 
270 

Cohn,  on  the  water  secreted  by 
Lathrifa  squamaria,  86,  n. ;  on 
the  movement  of  leaflets  of  Oxa- 
lis, 447 

Colutea  arborea,  nocturnal  move- 
ment of  leaflets,  355 

Coniferte,  circumnutation  of,  211 

Coronilla  rosea,  leaflets  asleep,  355 

Corylus  avellana,  circumnutation  of 
young  shoot,  emitted  from  the 
epicotyl,  55,  56 

,  arched  epicotyl,  77 

Cotyledon  umbilicus,  circumnuta- 
tion of  stolons,  219,  220 

Cotyledons,  rudimentary.  94-98  ; 
circumnutation  of,  109-112  ;  noc- 
turnal movements,  111,  112  ;  pul- 
vini or  joints  of,  112-122;  dis- 
turbed periodic  movements  by 
light,  123;  sensitiveness  of,  to 
contact,  125 ;  nyctitropic  move- 
ments of,  283,  297 ;  list  of  coty- 
ledons which  rise  or  sink  at 
night,  300 ;  concluding  remarks 
on  their  movements,  311 

Crambe  maritima,  circumnutation  of 
leaves,  228,  229 

Crinum  capense,  shape  of  leaves, 
253 


IXDEX. 


CRINUM. 

Crinum  capense,  circumnutation.  of, 
254 

Crotolaria  (sp.  ?),  sleep  of  leaves, 
310 

Cryptogams,  circumnutation  of, 
257-259 

Cucumis  dudaim,  movement  of  coty- 
ledons, 43,  44 

,  sleep  of  cotyledons,  304 

Cucurbita  aurantia,  movement  of 
hypocotyl,  42 

,  cotyledons  vertical  at  night, 

304 

ovifera,  geotropic  movement 

of  radicle,  38,  39 

,  circumnutation  of  arched  hypo- 
cotyl, 39 

,  of  straight  and  vertical  hypo- 
cotyl, 40 

,  movements  of  cotyledons,  41, 

42,  115,  124 

,  position  of  radicle,  89 

,  rupture  of  the  seed  -  coats, 

102 

,  circumnutation  of  hypocotyl 

when  erect,  107,  108 

,  sensitiveness  of  apex  of  radi- 
cle, 169-171 

,  cotyledons  vertical  at  night, 

304 

,  not  affected  by  apogeotropism, 

509 

,  tips  cauterised  transversely, 

537 

Curvature  of  the  radicle,  193 

Cycas  pectinata,  circumnutation  of 
young  leaf,  whilst  emerging  from 
the  ground,  58 

,  first  leaf  arched,  78 

,  circumnutation  of  terminal 

^leaflets,  252 

Cyclamt-n  Persicum,  movement  of 
cotyleilon,  46 

• ,  undeveloped  cotyledons,  78, 

96 

,  circumnutation  of  peduncle, 

225 

, ,  of  leaf,  246,  247 

,  downward  apheliotropic  move- 
ment of  a  flower  peduncle,  433- 
435 


DESMODIUM. 

Cyclamen  Persicum,  burying  of  the 

pods,  433 

Cyperus  alternifoUus,   circumnuta- 
tion of  stem,  212 

,  movement  of  stem,  509 

Cytisus  fragrans,  circumnutation  of 
hypocotyl,  37 

-,  sleep  of  leaves,  344,  397 

,    apogeotropic    movement    of 

stem,  494-496 


D. 


Dahlia,  circumnutation  of  young 
leaves,  244-246 

Dalea  alopecuroides,  leaflets  de- 
pressed at  night,  354 

Darkness,  effect  .of,  on  the  move- 
ment of  leaves,  407 

Darlingtonia  CaJifornica,  its  leaves 
or  pitchers  apheliotropic,  450,  n. 

Darwin,*.  Charles,  on  Maurandia 
semperflorens,  225 ;  on  the  Swedish 
turnip,  230,  n. ;  movements  of 
climbing  plants,  266.  271;  the 
heliotropic  movement  of  the  ten- 
drils ofBignonia  capreolata,  433  ; 
revolution  of  climbing  plants, 
451;  on  the  curling  of  a  tendril, 
570 

,  Erasmus,  on  the  peduncles  of 

Cyclamens,  433 

,  Francis,  on  the  radicle  of 

Sinapis  alba,  486 ;  on  Hygrosco- 
pic seeds,  489,  n. 

Datura  stramonium,  nocturnal 
movement  of  cotyledons,  298 

Delpino,  on  cotyledons  of  Chsero- 
pnyllum  and  Corydalis,  96,  n. 

Delphinium  nudicaule,  mode  of 
breaking  through  the  ground,  80 

,  confluent  petioles  of  two  coty- 
ledons, 553 

Desmodhim  gyrans,  movement  of 
leaflets,  '257,  ». 

,  position  of  leaves  at  night, 

285 

,  sleep  of  leaves,  not  of  coty- 
ledons, 314 

,    circumnutation    and    nycti- 


INDEX 


579 


DESMODIU3J. 

tropic  movement  of  leaves,  358- 
860 

Desmodium   gyrans,    movement   of 
lateral  leaflets,  361 
— ,  jerking  of  leaflets,  362 

,  nyctitropic  movement  of  peti- 
oles, 400,  401 

,  diameter  of  plant  at  night, 

402 

,  lateral  movement   of  leaves, 

404 

,  zigzag  movement  of  apex  of 

leaf,  405 

— ,  shape  of  lateral  leaflet,  416 

• vespertil/'onis,  364,  n. 

Deutzia  gracilis,  cireumnutation  of 
stem,  205 

Diageotropism,  5 ;  or  tramverse- 
geotropism,  520 

Diaheliotropism,  5;  or  Transversal- 
Heliotropismus  of  Frank,  419; 
influenced  by  epinasty,  439  ; 
by  weight  and  apogeotropirfin, 
440 

D/'anthus  carynplnjUm,  230 

,  cireumnutation  of  young  leaf, 

231,  269 

Dicotyledons,  cireumnutation  wide- 
ly spread  among,  68 

DioncBa,  oscillatory  movements  of 
leaves,  261,  271 

Dioncea  muscipula,  clrcumnutation 
of  young  expanding  leaf,  239, 
240 

• ,  closure  of  the  lobes  and  cir- 
eumnutation of  a  full-grown  leaf, 
241 

,  oscillations  of,  242-244 

Diurnal  sleep,  419 

Drosera  Capensis,  structure  of  first- 
formed  leaves,  414 

rolundifilia,     movement     of 

young  leaf,  237,  238 

,  of  the  tentacles,  239 

,    sensitiveness    of    tentacles, 

261 

,  shape  of  leaves,  414 

,  leaves  not  heliotropic,  450 

,  leaves  circumnutate  largely, 

454 

,  sensitiveness  of  570 


EUCALYPTUS. 

Duchartre  on  Tephrosia  cariboca, 
354;  on  the  nyctitropic  movement 
of  the  Cassia,  369 

Duval-Jouve,  on  the  movements  of 
Bryophyllum  calycinum,  237 ;  of 
the  narrow  leaves  of  the  Grami- 
nese,  413 

Dyer,  Mr.  Tinsel  ton,  on  the  leaves 
of  Crotolaria,  340  ;  on  Cassia  jlori- 
bunda,  3U9,  «.,  on  the  absorbent 
hairs  on  the  buried  flower-heads 
of  Trifolium  subterraneum,  517 


E. 


Echeveria  stolontfera,  cireumnuta- 
tion of  leaf,  237 

Echinocactus  viridescens,  its  rudi- 
mentary cotyledons,  97 

Echinocystis  lobata,  movements  of 
tendrils,  266 

,  apogeotropism  of  tendrils, 

510 

Elfving,  F.,  on  the  rhizomes  of 
Sparganium  ramosum,  189;  on 
the  diageotropic  movement  in  the 
rhizomes  of  some  plants,  521 

Elymus  arenareus,  leaves  closed 
during  the  day,  413 

Embryology  of  leaves,  414 

Engelmann,  Dr.,  on  the  Quercus 
virens,  85 

Epinasty,  5,  267 

Epk-otyl,  or  plumule,  5 ;  manner 
of  breaking  through  the  ground, 
77 ;  emerges  from  the  ground 
under  the  form  of  an  arch,  553 

Erythrina  caffra,  sleep  of  leaves, 
367 

corallodendron,  movement  of 

terminal  leaflet,  3b7 

crista-galli,    effect    of    tem- 
perature   on    sleep     of    leaves, 
318 

,  cireumnutation  and  nycti- 
tropic movement  of  terminal 
leaflets,  367 

Eucalyptus  resinifera,  cireumnuta- 
tion of  leaves,  244 


580 


INDEX. 


EUPHORBIA. 


Euphorbia     jacquineze flora,     nycti- 
tiopic  movement  of  leaves,  388 


F. 


of 


Flaliault,    M.,   on    the    rupture 
seed-coats,  102-104,"  106 

Flower-stems,  circumnutation  of, 
223-226 

Fragaria  Rosacea,  circumnutation 
of  stolon,  214-218 

Frank,  Dr.  A.  B.,  the  terms  Helio- 
tropism  and  Geotropism,  first 
used  by  him,  5,  n. ;  radicles  acted 
on  by  geotropism,  70,  n. ;  on  the 
stolons  of  Fragaria,  215;  periodic 
and  nyctitropic  movements  of 
leaves,  284;  on  the  root-leaves 
of  plants  kept  in  darkness,  443 ; 
on  pulvini,  485  ;  on  natural 
selection  in  connection  with 
geotropism,  heliotropism,  &c., 
570 

,    on    Transversal- Heliotropis- 

rnus,  419 

Fuchsia,  circumnutation  of  stem, 
205,  206 


G. 


circumnutation 


Gazania     ringens, 
of  stem,  208 

Genera  containing  sleeping  plants, 
320,  321 

Geotropism,  5 ;  effect  of,  on  the 
primary  radicle,  196 ;  the  reverse 
of  apogeotropism,  512  :  effect  on 
the  tips  of  radicles,  543 

Geranium  cinereum,  304 

Endressii,  304 

Ibericum,  nocturnal  movement 

of  cotyledons,  298 

Ricliardsoni,  304 

rotuudifolium,  nocturnal  move- 
ment of  cotyledon,  304,  312 

subcaulescens,  304 

Germinating  seed,  history  of  a, 
548 


GYMNOSFERMS. 

Gitliago  segetnm,  circumnutatiou  of 

hypocotyl,  21,  108 

,  burying  of  hypocotyl,  109 

,  seedlings  feebly  illuminated, 

124,  128 

,  sleep  of  cotyledon,  "02 

, leaves,  321 

Glaucinm    luteum,   circumnutation 

of  young  leaves,  228 
Gleditschia,  sleep  of  leaves   3-'5S 
Gli/cine  hispida,  vertical  si  iking  of 

haflets,  3G6 
Glycyrrhizn,  leaflets   depressed    at 

night,  355 
Godlewskl,    Emil,   on    the    turge- 

scence  of  the  cells,  485 
Gooseberry,  effect  of  radiation,  284 
Gossypium    (var.   Nankin    cotton), 

circumnutation     of      hypocotyl, 

22 

,  movement  of  cotyledon,  22.  23 

,  sleep  of  leaves,  324 

arboreum  (?),  sleep  of  cotylo- 

dons,  303 

Braziliense,  nocturnal  move- 
ment of  leaves,  324 

— ,  sleep  of  cotyledons,  303 

herbaceum,    sensitiveness     of 

apex  of  radicle,  168 

,  radicles  cauterised  trans- 
versely, 537 

maritimum,  nocturnal   move- 
ment of  leaves,  324 

Gravitation,  movements  excited  by, 
567 

Gray,  Asa,  on  Delphinium  nudi- 
caule,  80 ;  on  Megarrkiza  Cali- 
fornica,  8.1 ;  on  the  movements  in 
the  fruiting  fronts  of  Asplenium 
tricho manes,  257  ;  on  the  Amphi- 
carpcea  monoica,  520  ;  on  the 
Ipomoea  Jalappa,  557 

Grease,  effect  of,  on  radicles  and 
their  tips,  182,  185 

Gressner,  Dr.  H.,  on  the  cotyledons 
of  Cyclamen  Persicum,  46,  77 ; 
on  hypocotyl  of  the  same,  96 

Gymnosperms,  389 


INDEX. 


581 


HABERLANDT. 


H. 

Haberlandt,  Dr.,  on  the  protube- 
rance on  the  hypocotylof  Allium, 
59 ;  the  importance  of  the  arch 
to  seedling  plants,  87  ;  sub- 
aerial  and  subterranean  cotyle- 
dons, 110,  n. ;  the  arched  hypo- 
cotyl,  554 

HiBniuti.xylon  Campeclianum,  noc- 
turnal movement  of  leaves,  368, 
3(J9 

Hedrra  helix,  circumnutatiou  of 
stem,  207 

Hedysarum  coronarium,  nocturnal 
movements  of  leaves,  356 

Helianthemum  prostratum,  geotro- 
pic  movement  of  flower-heads, 
518 

Hebanihus  annuus,  circumnutation 
of  hypocotyl,  45 

,  arching  of  hypot-otyl,  90 

,  nocturnal  movement  of  coty- 
ledons, 305 

Heliotropism,  5  ;  uses  of,  449 ;  a 
modified  form  of  circumnutation, 
490 

SeUtiborus  niger,  mode  of  breaking 
through  the  ground,  86 

Hensen,  Prof.,  on  roots  in  worm- 
burrows,  72 

Henslow,  Kev.  G.,  on  the  coty- 
ledons of  thalaris  Canariensis. 
62 

Hofmeister,  on  the  curious  move- 
ment of  Spirogyrn,  3,  259,  n. ;  of 
the  leaves  ot  Pistia  utrutiotes, 
255 ;  of  cotyledons  at  night,  297 ; 
of  petals,  414 

and  B*talin  on  the  movements 

of  the  cabbage,  229 

Hooker,  Sir  J.,  on  the  effect  of  light 
on  the  pitchers  of  Sarracenia, 
450 

Hypocotyl,  5 ;  manner  of  break- 
ing through  the  ground,  77 ; 
emerges  under  the  form  of  an 
arch,  553 

Hypocotyls  and  Epicotyls,  circum- 


IPOMOJA. 

nutation  and  other  movements 
when  arched,9S;  power  of  straight- 
ening themselves,  100;  rupture 
of  the  seed-coats,  102-106  ;  illus- 
tration of,  106;  circuninut.tiion 
when  erect,  107;  when  in  dark, 
108 
Hyponasty,  6,  267 


I. 


Iberis  umbellata,  movement  of  stem, 
202. 

Illumination,  effect  of,  on  the  sleep 
of  leaves,  398 

Imatophyllum  vel  Clivia  (sp.  ?), 
movement  of  leaves,  255 

Indigofera  tinctoria,  leaflets  de- 
pressed at  night,  354 

Inheritance  in  plants,  407,  491 

Insectivorous  and  climbing  plants 
not  heliotiopic,  450  ;  influence  of 
light  on,  488 

Ipomaea  bona  nox,  arching  of  hypo- 
cotyl, 90 

,  nocturnal  position  of  coty- 
ledons, 306,  312 

ccerulea    vel    Fharbitis     nil, 

circumnutation      of      seedli.igs, 

,  movement  of  cotyledons    47— 

49,  109 

,  nocturnal  movements  of  coty- 
ledons, 305 

,  sleep  of  leaves,  386 

,  sensitiveness  to  light,  451 

,  the  hypocotyledonous   stems 

heliotropic,  453 

,  coccinea,  position  of  coty- 
ledons at  night,  306,  312 

leptophylla,  mode  of  breaking 

through  tt.e  ground,  83,  84 

,  arching  of  the  petioles  of  the 

cotyledons,  90 

,  difference  in  sensitiveness  to 

gravitation  in  different  parts, 
509 

,  extraordinary  manner  of  ger- 
mination, 557 


582 


INDEX. 


IPOMCEA. 

Tpomcea  pandurata,  n.auner  of  ger- 
iiiinati'  n,  84,  557 

purpurea  (vel  Pftorbttn    7i?-s- 

pida)j    nocturnal    movement    of 
cotyledons.  305,  312 

,  sleep  of  leave.-!,  386 

,  sensitiveness  to  light,  451 

,  the   hypocotyledonous   stems 

heliotropic,  453 

Iris  tMetKKMUwntt,  circumnutation 
of  leaves,  253 

Irniisch,  on  cotyledons  of  Ranun- 
culus Ficaria,  96 

Ivy,  its  stems  belio tropic,  451 


K. 


Kerner  on  the  bending  down  of  pe- 
duncles, 414 

Klinostat.  the,  an  instrument  de- 
vised by  Sachs  to  eliminate  geo- 
tropism,  93 

Kraus,  Dr.  Car],  on  the  underground 
shoots  of  Triticum  repens,  189; 
on  CannaUs  sativa,  250,  307, 
312  ;  on  the  movements  of  leaves, 
318 


Laduca  scar  tola,  sleep  of  cotyle- 
dons, 305 

Lagenaria  vulgaris,  circumnutation 
of  set-dlings,  42 

,  of  cotyledons,  43 

,  cotyledons  vertical  at  night, 

304 

Lathrxa  squamaria,  mode  of 
breaking  through  the  ground, 
85 

,  quantity  of  wafer  secreted, 

85,  86,  n. 

Liithyrus  nissolia,  circumnuta- 
tion of  stem  of  young  seedling, 
33 

,  ellipses  described  by,  107, 

108 

Leaves,    circumnutation    of,    226- 


LOTUS. 

262  ;  dicotyledons,  226-252  ;  mo- 
nocotyledons, 252-257 ;  nyctitro- 
pism  of,  28.) ;  their  temperature  af- 
fected l>y  their  position  at  night, 
294  ;  nyctitropic  or  sleep  move- 
ments, 315,  394 ;  periodicity  of 
their  movements  inherited,  407; 
embryology  of,  414 ;  s-j-called 
diurnal  sleep,  445 

Leguminosx,  sleep  of  cotyledons, 
308 ;  sleeping  species,  340 

Le  M-.tout  and  Decaisne,  67 

Lepidium  aativum,  sleep  of  cotyle- 
dons, 302 

Light,  movements  excited  by  418, 
563 ;  influence  on  mo.it  vegetable 
tissues,  486;  acts  on  plant  as  on 
the  nervous  system  of  animals, 
487 

Lilinm  auratum,  circumnutation  of 
stem,  212 

,  apogeotropic  movement  of 

stem,  498,  49J 

Linnaeus,  'Somnus  Plantarnm,' 
280;  on  plants  sleeping,  320; 
on  the  leaves  of  Slda  alwtilon, 
3J4;  on  CEnothera  mottissimcij 
383 

Linum  Berendieri,  nocturnal  move- 
ment of  cotyledons,  298 

usitat/ssimum,  circumnutation 

of  stem,  203 

Loli'tm  perenne,  joints  affected  by 
apogeotropisin,  502 

Lonicera  brachypoda,  hooking  of  the 
tip,  272 

,  sensitiveness  to  light.  453 

Loomis,  Mr.,  on  the  movements  in 
the  fruiting  fronds  of  Asplenium 
trichomanes,  257 

Lotus  aristata,  effect  of  radiation 
on  leaves,  292 

Creticus,  leaves   awake    and 

asleep,  354 

Gtbelii,  nocturnal   movement 

of  cotyledons,  308 

,  leaflets  provided  with  pulvini, 

353 

Jacobxus,  movements  of  coty- 
ledons, 35,  109 

,  pulvini  of,  115 


IXDEX. 


583 


LOTUS. 

Lotus     Jacobxus,     movements    nt 

night,  116,  121,  124 

• ,  development  of  pulvini,  122 

,  sleep  of  cotyledons,  308,  313 

.     nyctitropic     movement     of 

leaves,  353 
major,  sleep  of  leaves,  353 

perigrinus,  movement  of  leaf- 
lets, 353 

Lumdaria  vnlgaris,  circumnutation 

of  fronds,  258 
Lupinu*.  340 

—  albifrons,  sleep  of  leaves,  344 

Hartwegii,   sleep    of    leaves, 

341 

lateus,  circunmutation  of  coty- 
ledons, 3S,  110 

,  effect  of  darkness,  1 24 

Lupinus,  position   of  leaves    when 
asleep,  341 

,  different  positions  of  leaves  at 

night,  343 

,  varied  movements  of  leaves 

arid  leaflets,  395 

Menziesii,  sleep  of  leaves,  343 

mutabilis,    sleep    of   leaves, 

•343 

names,  sleep  of  leaves,  343 

p/losus,  sleep  of  leaves,  340, 


341 

343 


sleep   of  leaves, 


sleep  of  leaves  by 
d;ty  and  night,  342 

,  position  of  petioles  at  night, 


34:  { 

—  ,  movements  of  petioles,  401 
-  -  speciosus,    circumuutation   of 

leaves,  236 
Lynr;h,  Mr.  R.,  on  Pacliira  aqua- 

tica,  95,  n.  ;   sleep  movements  of 

Averrhoa,  330 


Maranta  arundlnacea,  nyctitropic 
movement  of  leaves,  389-391 

,  after  much  agitation  do  not 

sleep,  319 


MELILOTUS. 

Marsilia  quadrifnliata,  effect  of  ra- 
diation at  night,  292 

• ,  circnmnntatiori  and  nycti- 
tropic movement  of  leaflets,  3U2- 
3J4 

,  rate  of  movement,  404 

Martins,  on  radiation  at  night, 
284,  n. 

Master?,  Dr.  Maxwell,  on  the  lead- 
ing shoots  of  the  Coniferx,  211 

Maurandia  semperflorem,  circumnu- 
tation  of  peduncle,  225 

Medicago  maculata,  nocturnal  posi- 
tion of  leaves,  345 

marina,  leaves    awake     and 

asleep,  344 

Meehan,  Mr.,  on  the  effect  of  an 
JEcidium  on  Portulaca  oleracea, 
189 

Megarrhiza  Calffornica,  mode  of 
breaking  through  the  ground, 
81 

,  germination  described  by  Asa 

Gray,  82 

,  fcingular  manner  of  germina- 
tion, 83,  556 

Melal-Mca  ericce/olia,  sleep  of  leaves, 
383 

Melilotus,  sleep  of  leaves,  345 

alba,  sleep  of  leaves,  347 

ccerulea,  sleep  of  leaves,  347 

dentata,  effect  of  radiation  at 

night,  295 

elegans,  sleep  of  leaves,  347 

gracilis,  sleep  of  leaves,  347 

infesta,  sleep  of  leaves,  347 

Italica,    leaves    exposed    at 

night,  291 

,  sleep  of  leaves,  347 

macrorrhiza,  leaves  exposed  at 

night,  292 

,  sleep  of  leaves,  347 

messanensis,  sleep  of  leaves  on 

full-grown    and    young    plants, 

348,  416 
ojficinalis,  effect  of  exposure  of 

leaves  at  night,  21)0,  296 
,  nocturnal  movement  of  leaves, 

346,  347 

,  circumnutation  of  leaves,  348 

,  movement  of  petioles,  401 


58-i 


INDEX. 


MLLILOTUS. 

Melihius  parriflora,  sleep  of  leaves, 
317 

Pttitpierreana,  leaves  exposed 

at  night,  291,  296 

,  sleep  of  leaves,  347 

secuiulijiora,  sleep  of  leaves, 

347 

tuaveolens,  leaves  exposed  at 

night,  291 

,  sleep  of  leaves,  347 

sulcata,  sleep  of  leaves,  347 

Taurica,   leaves   exposed    at 

night,  291 

— ,  sleep  of  leaves,  347,  415 
Methods  of  observation,  6 
Mimosa  albida,  cotyledons  vertical 

at  night,  116 

,  not  sensitive  to  contact,  127 

,  sleep  of  cotyledons,  308 

,  rudimentary  leaflets,  3d4 

,    nyctitropic    movements     of 

leaver,  379,  380 
,  circunmutation  of  the  main 

petiole  of  young  leaf,  381 
,  torsion,  or  rotation  of  leaves 

and  leaflets,  400 

,  first  true  leaf,  416 

,  effect  of  brght  sunshine  en 

basal  leaflets,  445 

maryinata,  nyctitropic  move- 
ments of  leaflets,*  381 

pudica,   movement  of    coty- 
ledons, 105 

,  rupture    of   the    seed-coats, 

105 
,  circumnutation  of  cotyledons, 

109 

,  pulvini  of,  113,  115 

,  cotyledons  vertical  at  night, 

116 
,  hardly  sensitive  to  contact, 

127 
,  effect  of  exposure  at   night, 

293 
,  nocturnal  movement  of  leaves, 

297 

,  sleep  of  cotyledons,  308 

,    circumnutation    and    nycti- 
tropic movement  of  main  petiole, 

374-378 
,  of  leaflets,  378 


NEPTUNIA. 

Mimosa  albida,  circumnutation  and 
nyctitropic  movement  of  pinnae, 
402 

,  number  of  ellipses  described 

in  given  time,  406 

,  effect  of  bright  sunshine  on 

leaflets,  446 

Mirabilis  jalapa  and  longiflora, 
nocturnal  movements  of  cotyle- 
dons, 307 

,  nyctitropic  movement  of 

leaves,'  387 

Mohl,  on  heliotropism  in  ten- 
drils, stems,  and  twining  plants, 
451 

Momentum-like  movement,  the  nc- 
cumnlated  effects  of  apogeo- 
tropi-m,  508 

Monocotyledons,  sleep  of  leaves, 
389 

Monotropa  hypripifys,  mode  of 
breaking  through  the  ground,  86 

Mori-en,  on  the  movements  of 
stamens  of  Sparmanuia  and 
Cereus,  226 

Miiller,  Fritz,  on  Cassia  fora,  34; 
on  the  circumnutation  of  Linum 
usitotissimum,  203;  movements 
of  the  flower-stems  of  an  Alisma, 
226 

Mutisia  cUmatis^  movement  of 
leaves,  246 

,  leaves  not  heliotropic,  451 


N. 


Natural  selection  in  connection 
with  geotropism,  heliotropism, 
&c.,  570 

Nephrodium  molle,  circumnutation 
of  very  young  frond,  63 

,  of  older  frond,  257 

,  slight  movement  of  fronds, 

509 

Neptunia  oleracea,  sensitiveness  to 
contact,  128 

,  nyctitropic  movement  of  leaf- 
lets, 374 

,  of  pinnae,  402 


INDEX. 


585 


NICOTIAN  A. 

Nicotiana  glauca,  sleep  of  leaves, 
385,  380' 

,  circumnutation  of  leaves, 

886 

Nobbe,  on  the  rupture  of  the  seed- 
cuats  in  a  seedling  of  Martynia, 
105 

Nolana  prostrata,  movement  of  seed- 
lings in  the  dark,  50 

,  tircunmutution  of  seedling, 

108 

Nycti  tropic  movement  of  leaves, 
500 

Nyctitropism,  or  sloop  of  leaves, 
281 ;  in  connection  with  radia- 
tion, 286;  object  gained  by  it, 


0. 


Observati  n,  methods  of,  6 

(Enothera  rnollifsima,  sleep  of  leaves, 
383 

Opuntia  basilar/s,  conjoint  circum- 
nutation of  hypocotyl  and  coty- 
ledon, 44 

,  thickening  of  the  hypocotyl, 

96 

,  circumnutation  of  hypocotyl 

when,  erect,  J07 

,  burying  of,  109 

Orange,  seedling,  circumuutation 
of,  510 

Orchis  pyramidalis,  complex  move- 
ment of  pollinia,  489 

Oxalis  acetosella,  circumnutation  of 
flower-stem,  224 

,  effects  of  exposure  to  radia- 
tion at  night,  287,  288,  296 

,  circumnutation  and  nycti- 

tropic  movement  in  full-grown 
leaf,  326 

,  circumnutation  of  leaflet  when 

asleep,  327 

.  rate  of  circumnutation  of 

leaflets,  404 

• ,  effect  of  sunshine  on  leaflets, 

417 

• ,  circumnutation  of  peduncle, 

506 


OXALIS. 

Oxalis  acetosella,  seed-capsules,  only 
occasionally  buried,  f>18 

-    articulala,    nocturnal    move- 
ments of  cotyledons,  807 

{Biopliytum}     semdiva,    ra- 
pidity of  movement  of  cotyledons 
during  the  day,  26 

,  pulvinus  of,  113 

,  cotyledons  vertical  at  night, 

116,  118 

bupleurifolia,   circumnutation 

of  foliaceous  petiole,  328 

,  nyctitropic  movement  of  ter- 
minal leaflet,  329 

carnosa,    circumnutation    of 

flower-stem,  223 

,  epinastic  movements  of  flower- 
stem,  504 

,  effect  of  exposure  at  night, 

288,  296 

,  movements  of  the  flower-pe- 
duncles due  to  apogtotropism 
and  other  forces,  503-506 

corniculata     (var.     cuprea}, 

movements  of  cotyledons,  26 

,  rising  of  cotyledons,  116 

,  rudimentary  pulvini  of  coty- 
ledons, 119 

• ,  development  of  pulvinus, 

122 

,  effect  of  dull  light,  124 

,  experiments  on  leaves  at  night, 

288 

floribunda,  pulvinus  of  coty- 
ledons, 114 

.  nocturnal  movement,  118, 

307,  313 

fragrans,    sleep     of    leaver, 

324 

• Ortegesii,  circumnutation  of 

flower  stems,  224 

,  sleep  of  large  leaves,  327 

,  diameter  of  plant  at  night, 

402 

,  large  leaflets  affected  by  bright 

sunshine,  4-17 

Plumierii,  sleep  of  leaves,  327 

purpurea,  exposure  of  leaflets 

at  night,  293 

rosea,  circumnutation  of  coty- 
ledons, 23,  24 


586 


OXALIS. 

Oxalis  rnsea,  pulvinus  of,  113 
,   movement    of  cotvledons   at 

night.  117,  118.307 

,  effect  of  dull  light,  124 

,      uon  -  sensitive      cotyledon*, 

127 

sensitiva,  movement  of  coty- 
ledons, 100,  127, 128 

,  circumnutatioii  of  flower-stem, 

224 

,  nocturnal  movement  of  coty- 
ledons, 307,  312 

,  sleep  of  leaves,  327 

tropceoloides,  movement  of  co- 
tyledons at  night,  118, 1'-dO 

Valdiciana,  conjoint  cireum- 

nutaticm  of  cotyledons  and  hypo- 
cotyl,  25 

,  cotyledons  rising  vertically  at 

night,  114,  115,  117,  118 

,  non-sensitive  cotyledons,  127 

,  nocturnal  movement  of  coty- 
ledon, 307,  312 

,  sleep  of  leaves  and  not  of  co- 
tyledons, 315 

-,  movements  of  leaves,  327 


P. 


Pacliira  aquatica,   unequal  cotyle- 
dons, Oo,  n. 
Pancratium  liltorale,  movement   of 

leaves,  2.55 
Paraheliotropism,  or  diurnal  sleep 

of  leaves,  445 
Pastiflura  (jracilis,  circumnutation 

and    nyctitropic    movement     of 

K  aves,  383, 3»4 
,    apogeo tropic    movement    of 

tendrils,  5 10 

,  sensitiveness  of  tendrils,  550 

Pelargonium  zonale,  circumuutation 

of  stem,  203 
,  and  downward   movement  of 

young  leaf,  232,  233,  260 
Petioles,  the,  rising  of,  beneficial  to 

plant  at  night,  402 
Petunia  violacea,  downward  move- 


niASKOLUS. 

ment  and  cireummitation  of  very 
young  leaf,  24S,  249,  26'J. 

Pfcffer,  Prof.,  on  the  tuvgescence  of 
the  cells,  2  ;  on  pulviui  of  K  aves, 
113,  117;  sleep  movements  of 
haves.  280,  283,  "281;  nocturnal 
rising  of  leaves  of  Malva,  324  ; 
movements  of  leaflets  in  J)esir.o- 
dium  py  rans,  358;  on  Phijllan- 
thns  Niruri,  :~'88;  influence  of  a 
pulvinus  on  leaves,  3-»G;  periodic 
movements  of  sleeping  leaves, 
407,  40* ;  movements  of  petals, 
414;  effect  of  bright  sunshine  on 
leaflets  of  Robinia.  445;  effect  of 
light  on  parts  provided  with  pul- 
vini,  3G3 

Phalaris  Canariensis,  movements  of 
old  seedlings,  02 

,  circumnutation  of  cotyledons, 

G3,  64,  108 

,  heliotropic  movement  and  cir- 
cumnutation of  cotyledon  towards 
a  dim  lateral  light,"  427 

,  sensitiveness  of  cotyledon  to 

light,  455 

,   effect  of  exclusion   of  light 

from  tips  of  cotyledons,  45G 

,  manner  of  bending   towards 

light,  457 

,  effects  of  painting  with  Indian 

ink,  467 

,  transmitted   effects  of  light, 

469 

,   lateral    illumination   of   tip, 

470 

,  apngsotropic  movement  of  the 

sheath-like  cotyLdons,  497 

,  change  from  a  straight  up- 
ward apogeotropic  course  to  cir- 
cumnutation, 499 

,    apogeotropic     movement    of 

cotyledons,  500 

Phaseolus  Hernandesii,  nocturnal 
movement  of  leaves  and  leaflets, 
3U8 

caracalla,  93 

,  nocturnal  movement  of  leaves, 

368 

— — ,  effect  of  bright  sunshine  on 
leaflets,  446 


INDEX. 


587 


rilASEOLUS. 

Phaseolus  multiflorus,  movement  of 

radicles,  29 

,  of  young  radicle,  72 

,  of  hypocotyl,  91,  93 

,  sensitiveness  of  apex  of  radicle, 

163-167 

,  to  moist  air,  181 

,  cauterisation   and  grease  on 

the  tips,  535 
,  nocturnal  movement  of  leaves, 

368 
,  nyctitropic  movement   of  the 

fiist  unifoli,»te  leaves,  397 

lloxburghii,   effect   of  bright 

(sunshine  on  first  leaves,  415 

,  vulgarin,  93 

,  bleep  of  leaves*  31 8 

,  vertical  sinking  of  leaflets  at 

night,  'MS 

Phyllanthus  Niruri,  sleep  of  leaf- 
lets, 388 

linoides,     sleep     of    leaves, 

387 

Pilocereus    Hnulletii,    rudimentary 

cotyledon?,  97 
Pimelia  spectabilis,  sleep  of  leaves, 

387 
Pincers,    wooden,    through    which 

the  radicle  of  a  bean  was  allowed 

to  grow,  75 
Pinits  austriaca.  circunmutation  of 

leaves,  '25 1,  252 

Nordinanniana,     nyctitropic 

movement  of  leaves,  389 

pinaster,    circunmutation    of 

hyp;icotyl.  5(j 

,   movement  of    two    opposite 

cotyledons,  57 
,  cii  cumnutation  of  young  leaf, 

250,  251 

,    epinastic    downward    move- 
ment of  young  leaf,  270 
Pistia    stmtiutes,      movement      of 

leaves,  255 
Piaum    satii'um,    sensitiveness     of 

apex  of  radicle,  158 
,   tips    of   radicles    cauterised 

transversely,  531 
Plants,      sensitiveness      to      light, 

4-19;  hygroscopic  movements  of, 

489 


QUEKCUS. 

Plants,  climbing,  circunmutation  of, 
261 ;  movements  of,  55l> 

,  mature,  circuumutation  of, 

201-214 

Pliny  on  the  sleep-movements  of 
plants,  28 ) 

Plumbago  Capensis,  circumnutation 
of  stem,  208,  209 

Poinciana  Gilliesii,  sleep  of  leaves, 
368 

Polygonum  aviculare,  leaves  vertical 
at  night,  387 

1  convolvulus,  sinking  of  the 

leaves  at  night,  318 

Pontederia  (sp.  V),  circunmutation 
of  leaves,  256 

Porlieria  hygrometrica,  circum- 
nutation an1 1  nyctitropic  move- 
ments of  petiole  of  leaf,  335, 
336 

,  effect  of  watering,  336-338 

,  leaflets  closed  during  the  cliy, 

413 

Portnlaca  oleracea,  effect  of  Mci- 
dium  on,  189 

Primula  Sinensis,  conjoint  circum- 
nutation of  hypocotyl  and  coty- 
ledon, 45,  4H 

Pringsheim  on  the  inj  ury  to  chloro- 
phyll,  446 

Prosopis,  nyctitropic  movements  of 
leaflets,  374 

Psoralea  acaulis,  nocturnal  move- 
ments of  leaflets,  354 

Pteris  aquiUua,  i-achis  of,  86 

Pulvini,  or  joints;  of  cotyledons, 
112-122;  influence  of,  on  the 
movements  of  cotyledons,  313; 
effect  on  nyctitropic  movements, 
396 


Qttercm  (American  sp.),  circurnuu- 
tation  of  young  stem,  53,  54 

robur,  movement  of  radicles, 

54,  55 

,  sensitiveness  of  apex  of 

radicle,  174-176 


588 


INDEX. 


Quercus  rirem,  manner  of  germina- 
tion, 85,  557 


Radiation  at  night,  effect  of,  0:1 
b  aves,  284-286 

Radicles,  manner  in  which  they 
penetrate  the  ground,  69-77 ;  cir- 
cumnutation  of,  69  ;  experimenfs 
with  split  sticks,  74 ;  with 
wooden  pincers,  75;  sensitiveness 
of  apex  to  contact  and  other  irri- 
tants, 129  ;  of  Vicia  fdba,  132- 
15S;  various  experiments,  135- 
140;  summary  of  results.  143-151; 
power  of  an  irritant  on,  com- 
pared with  geotropism,  151-154; 
sensitiveness  of  tip  to  moist 
air,  180 ;  with  greased  tips, 
185 ;  effect  of  killing  or  injuring 
the  primary  radicle,  187-191 ; 
curvature  of,  193;  affected  by 
moisture,  198;  tip  alone  sensitive 
to  geotropism,  540;  protrusion 
and  circumnutation  in  a  germina- 
ting seed,  548;  tip  highly  sen- 
sitive, 550;  the  tip  acts  like  the 
brain  of  one  of  the  lower  animals, 
573 

,  secondary,  sensitiveness  of 

the  tips  in  the  bean,  lot ;  become 
vertically  geotiopic,  186-191 

Barney  on  the  movements  of  the 
cotyledons  of  Mimosa  pudica, 
&m\  Clianthus  Dampieri  at  night, 
297 

Ranunculus  Ficaria,  mode  of 
breaking  through  the  gr.und, 
86,  90 

,  *  ingle  cotyledon,  96 

,  effi  ct  of  lateral  light,  484 

Raplianus  Kativa,  s  nsitiveucss  of 
apex  of  radicle,  171 

,  sleep  of  cotyledons,  301 

Rattan,  Mr.,  on  the  germination  of 
the  seeds  of  Megarrhi.a  Califor- 
nia, 82 

Relation  between  circumnutation 
and  heliotropism,  435 


SACHS. 

Reseda  odorata,  hypocotyl  of  seed- 
ling slightly  heliotrnpic,  454 

Reversion,  due  to  mutilation,  190 

Rhipsdlis  cassytha,  rudimentary  co- 
tyledons, 97 

Ricinns  Borboniens/s,  circumnuta- 
i  tion  of  arched  hypocotyl,  53 

Robinia,  effect  of  bright  sunshine 
on  its  leaves,  445 

pseudo-ctcacia,  leaflets  vertical 

at  night,  355 

Rodier,  M.,  on  the  movements  of 
Ceratnphyllum  dem^rsum,  211 

Royer,  Cn.,on  the  sleep-movements 
of  plants.  281,  n. ;  on  the  sle  p  of 
leaves,  318  :  the  leaves  of  Mnli- 
cago  maculatd,  345  ;  on  Wistaria 
Sinensis,  354 

Rubus  id&us  (hybrid)  circumnuta- 
tion of  stem,  205 

,  apogeotropic  movement  of 

stem,  498 

Ruiz  and  Pavon,  on  Porlier.'a  hy- 
(jrometrii-a,  336 


SACHS  on  "  revolving  nutation,"  1  ; 
intimate  connection  between  tur- 
gescence  and  growth,  2,  n. ;  coty- 
ledon of  the  onion,  59 ;  adapta- 
tion of  root-hairs,  69  ;  the  move- 
ment of  the  rad.cle,  70,  72,  73; 
movement  in  the  hypocotyls  of 
the  bean,  &c,  91  ;  sensitiveness 
of  radicles.  131,  145,  198;  sensi- 
tiveness of  the  primary  radicle 
in  the  bean,  155.;  in  the  com- 
mon pea,  156 ;  effect  of  moist 
air,  180;  of  killing  or  injuring 
the  primary  radicle,  186,  187 ; 
circumnutation  of  flower-stems, 
225;  epinasty,  2G8 ;  movements 
of  leaflets  of  Trifolium  incar- 
natum,  350;  action  of  li^ht  in 
modifying  the  periodic  move- 
ments of  leaves,  418  ;  on  geotro- 
pism and  heliotropism,  436,  ». ; 
on  Tropceolum  majut,  453 ; 


INDEX. 


589 


SARRACENIA. 

on  the  hypocotyls  slightly  helio- 
tropic,  and  stems  strongly  aphe- 
liotropic  of  the  ivy,  453 ;  lie- 
liotropism  of  radicles,  482  ;  ex- 
periments on  tips  of  ra-iieles 
of  b  -an,  523,  524  ;  curvature  of 
tho  hypocotyl,  555  ;  resemblance 
bit  we  en  plants  and  animals, 
571 

Sarracenia  purpurea,  cirmmnuta- 
tion  of  young  pitcher,  227 

Saxifrnga  sarmentosa,  circum- 
nutation  of  an  inclined  stolon, 
218 

Scliranlda  aculeata,  nyetitropic 
movement  of  the  pinna?,  381, 
403 

uncinata,  nyctitropic  move- 
ments of  leaflets.  381 

Securigera  cornnilla,  nocturnal 
movements  of  leaflets,  352 

Seed-capsules,  burying  of,  513 

Seed-coats,  rupture  of,  102-106 

Seedling  plants,  ciivuinnutating 
movements  of,  10 

lti,  circumnutation  of,  258 
Kraussii  (?),  circumnutation  of 
young  plant,  6(5 

Sida  napoea,  depre^sio.i  of  leaves  at 
night,  3'22 

,  no  pulvinus,  322 

retusa,  vertical  rising  of  leaves, 

322 

rhomb  'folia,  sleep  of  cotyledons, 

308 

,  sleep  of  leaves,  314 

,  vertical  rising  of  leaves,  322 

,  no  pulvinus,  322 

,    circumnutation.     and    nycti- 

tropic  movements  of  leaf  of  young 
plant,  322 

,    nyctitropio     movement     of 

leaves,  897 

Siegesbeclua  orientally  sleep  of 
leaves,  310,  384 

Sinapis  alba,  hypocotyl  bending  to- 
wards the  light,  4G1 

,  transmitted  effect  of  light  on 

radicles,  482,  483,  567 

,  growth  of  radicles  in  dark- 
ness, 486 


STAPELTA. 

Sinapis  nigra,  sleep  of  cotyledons, 

30  L 
Smilax    aspera,    tendrils    aphelio- 

tropic,  451 
Smiihia     Pfundii,     non  -  sensitive 

cotyledons,  127 
,  hyponastic  movement  of  the 

curved  summit  of  the  stem,  274- 

27G 
,   cotyledons    not    sleeping    at 

night,  308 
,  vertical  movement  of  leaves, 

356 

sensitira,  sensitiveness  of  coty- 
ledons to  contact,  126 

,  sleep  of  cotyledons,  308 

Sophora  chrysophylla  leaflets  rise  at 

night,  368 
Solanum    dulcamara,    circumnuta- 

ting  i-tom?,  266 

lycopersicum,    movement    of 

hypocotyl,  50 

,  of  cotyledons,  50 

,  effect  of  darkness,  124 

,  rising  of  cotyledons  at  night. 

306 
,    heliotrop'c     movement*     of 

hypocotyl,  421 
,  effect  of  an  intermittent  1'ght. 

457 
,  lapid  hcliotropi^m,  461 

pal/'nacanthum,     oircumnu- 

tatiou   of  arched    hypocotyl,  51, 
100 

— — ,  of  cotyledon,  51 

,  ellipses  described  by  hypo- 
cotyl when  erect,  107 

,  nocturnal  movement  of  cotv- 

ledons,  306 

Sparganium  ramosum,  rhizomes  of 
189 

Sphcerophysa  salsola,  rising  of 
leaflets,  355 

Spiroijyra  princeps,  movements  of, 
259,  «. 

Stall,  Dr.,  on  the  effect  of  JEoi- 
dium  on  shoot,  189;  on  the  in- 
fluence of  light  on  swarm-spores, 
488,  n. 

Stapdia  sarpedon,  circumnutatiou 
of  hypocotyl,  46,  47 


590 


IXDEX. 


STAFEL1A. 

Stapelia  sarpedon,  minute  coty- 
ledons, 97 

Stellar ia  media,  nocturnal  move- 
ment of  leaves,  297 

Stems,  circumnutation  of,  201-214 

Stolons,  or  Runners,  circumnuta- 
tion of,  214-222,  558 

Stnisbuiger,  on  the  effect  of  light 
on  spores  of  Hsematoc-cus,  455,  n.  ; 
the  influence  of  light  on  t<.e 
swarm-spores,  488 

Strawberry,  stolons  of  the,  circum- 
nutiifc,  but  not  affected  by  mode- 
rote  light,  454 

Striphium  jloribundum,  circumnu- 
tation and  nyclitropic  movement 
of  leaves,  391,  392 


T. 


Tamarindus  Indica,  nyctitropic 
movement  of  leaflets,  374 

Transversal  -  heliotrop^mus  (of 
Frank)  or  diaheliotropistn,  438 

Trcpa  natans,  unequal  cotyledons, 
95,  n. 

Tecoma  radicans,  stems  aphelio- 
tropic,  451 

Tephrosia  caribaa,  35 1 

Terminology,  5 

TJialia  dealbata,  sleep  of  leaves, 
389 

,  lateral  movement  of  leaves, 

404 

TricJiosantJies  anyiiina,  action  of  the 
peg  on  the  radicle,  104 

,  nocturnal  movement  of  coty- 
ledons, 304 

Tr i  folium,  position  of  terminal  leaf- 
lets at  night,  282 

globoisum,  with  hairs  protecting 

the  seed-bearing  flowers,  517 

glomeratum,    movement    of 

cotyledons,  309 

incarnatum,    movement    of 

cotyledons,  309 

Pannonicum,   shape  of    first 

true  leaf,  350,  4J5 


1  RITICUM. 

Tri folium  pratense,  leaves  exposed 
at  night,  293 

repens,     circumnutation     of 

flower-stem,  225 

,  circumnutatingand  epinastic 

movements  of  flower-stem,  276- 

-  279 

,  nyctitropic  movement  of 

leaves,  349 

,  circumnutation  and  nycti- 
tropic movements  of  terminal 
leaflets,  352,  353 

,  sleep  movements,  349 

resupinatum,    no    pulvini    to 

cotyledons,  118 

,  circumnutation  of  stem,  204 

,  effect  of  exposure  at  night, 

295 

— ,     cotyledons    not     rising    at. 
night,  118,  3d9 

,  circumnutation  and  nycti- 
tropic movements  of  terminal 
leaflets,  351,  352 

strictum,  movements  of  coty- 
ledons at  night,  110,  118 

,  nocturnal  and  diurnal  move- 
ments of  cotyledons,  309-311, 
313 

,  movement  of  the  left-hand 

cotyledon,  316 

aubterraneum,    movement    of 

flower-heads,  71 

,  of  cotyledons  at  night,   116, 

118,309 
,  circumnutation  of  flower-stem, 

224,  225 

,   eircumnutation    and    nycti- 
tropic movements  of  leaves,  350 
,  number    of    ellipses    in    24 

hours,  405 
,  burying  its  fhwcr  he  ids,  513, 

514 
,  downward  movement  of  pi- 

duncle,  515 
.  circumnutating  movement  01 

peduncle,  51t> 
Trigonella  Cretica,  sleep  of  leaves, 

345 
Triticnm       repens,       underground 

shoots  of,   become   apogeotropie, 

189 


INDEX. 


591 


TRITICUM. 

Triticum  vulgare,   sensitiveness    of 

tips  of  radicle  to  moist  air,  181 
Tropce.olum  majus  (?),  sensitiveness 

of  apex  of  radicle  to  contact,  167 

,  circumnutation  of  stem,  204 

,  influence  of  illumination  on 

nyctitropic  movements,  838-340, 

344 
,    heliotropic    movement    and 

circumnutation  of  epicotyl  of  a 

young  seedling,  428,  429 
• ,  of  an  old  internode  towards  a 

lateral  light,  430 
,  stems  of  very  young  plants 

highly  heliotropic,  of  old  plants 

slightly  apheliotropic,  453 

,  effect  of  lateral  light,  484 

minus  (?),  circumnutation   of 

buried  and  arched  epicotyl,  27 


Ulex,  or  gorse,  first-formed  leaf  of, 
415 

Uraria  lagopus,  vertical  sinking  of 
leaflets  at  night,  365 

Vaucher,  on  the  burying  of  the 
flower-heads  of  Tri folium  sub- 
terraneurn,  513;  on  the  protec- 
tion of  seeds,  517 

Verbena  melindres  (?),  circumnuta- 
tion of  stem,  210 

,  apogeotropic  movement  of 

stem,  495 


V. 


Vicia  faba,  circumnutation  of  ra- 
dicle, 29,  30 

,  of  epicotyl,  31-33 

,  curvature  of  hypocotyl,  92 

,  sensitiveness  of  apex  of  ra- 
dicle, 132-134 

,  of  the  tips  of  secondary  ra- 
dicles, 154 

,  of  the  primary  radicle  above 

the  apex,  155-158 

,  various  experiments,   135-143 

,  summary  of  results,  143-151 

,  power  of  an  irritant  on,  com- 

26 


WILSON. 

pared   with   that   of  geotropism, 
151-154 

Vicia  faba,  circumnutation  of  leaves, 
233-235 

,   circumnutation   of    terminal 

leaflet,  235 

,  effect  of  apogeotropism,  444 

,  effect  of  amputating  the  tips 

of  radicles,  523 

,  regeneration  of  tips,  526 

,  short    exposure   to  geotropic 

action,  527 

,  effects  of  amputating  the  tips 

obliquely,  528 

,  of  cauterising  the  tips,  529 

,  of  grease  on  the  tips,  534 

Vines,  Mr.,  on  cell  growth,  3 
Vries,  De,  on  turgescence,  2 ;  on 
epinasty  and  hyponasty,  6,  267, 
268 ;  the  protection  of  hypo- 
cotyls  during  winter,  557  ;  stolons 
apheliotropic,  108 ;  the  nycti- 
tropic movement  of  leaves,  283 ; 
the  position  of  leaves  influenced 
by  epinasty,  their  own  weight  and 
apogeotropism,  440 ;  apogeotro- 
pism in  pt-tioles  and  midribs,  443  ; 
the  stolons  of  strawberries,  451 ; 
the  joints  or  pulvini  of  the  Gra- 
minese,  502 

W. 

Watering,  effect  of,  on  Forlieria 
hygrometrica,  336-338 

Wells,  « Essay  on  Dew,'  284,  n. 

Wiesner,  Prof.,  on  the  circumnuta- 
tion of  the  hypocotyl,  99,  100  ; 
on  the  hooked  tip  of  climbing 
stems,  272 ;  observations  on  the 
effect  of  bright  sunshine  on 
chlorophyll  in  leaves,  446;  the 
effects  of  an  intermittent  light, 
457 ;  on  aerial  roots,  486 ;  on 
special  adaptations,  490 

Wigandia,  movement  of  leaves,  248 

Williamson,  Prof,  on  leaves  of 
Drosera  Capensis,  414 

Wilson,  Mr.  A.  S.,  on  the  move- 
ments of  Swedish  turnip  leaves. 
230,  298 


592 


INDEX. 


Winkler  on  the  protection  of  seed- 
lings, 108 

Wistaria  Sinensis,  leaflets  depressed 
at  night,  351 

,  circumnutation  with  lateral 

light.  452 


Zea  Mays,  circumnutation  of  coty- 
ledon, 64 


ZUKAL. 

Zea  Mays,  geotropic  movement  of 
radicles,  C5 

,  sensitiveness  of  apex  of  ra- 
dicle to  contact,  177-179 

,  secondary  radicles,  179 

,  heliotropic  movements  of 

seedlinjr,  64,421 

,  tips  of  radicles  cauterised, 

539 

Zukal,  on  the  movements  of  Spira- 
lina,  259,  n. 


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THE    HISTORICAL    POETRY    OF    THE    ANCIENT     HEBREWS. 

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some  of  the  greatest  questions  of  philosophy  in  their  most  recent  forms,  as  set  or 
reset  by  the  last  speculations  and  revelations  of  science,  has  already  shot  his 
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WORKS  OF  HENRY  THOMAS  BOCKLE. 


The   Life  and  Writings   of   Henry 
Thomas  Buckle. 

By  ALFRED  HENRY  HUTH.     12mo.     Cloth. 

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"Mr.  Hutli  has  produced  a  striking  and  distinct  portrait  out  of  his  materials, 
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History  of  Civilization  in  England. 

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for  a  good  deal.  Let  the  reader  be  as  adverse  as  he  may  be  to  the  water's  philos- 
ophy, let  lii:n  be  as  devoted  to  the  obstructive  as  Mr.  Buckle  is>  to  the  progress 
party,  let  him  be  as  orthodox  in  church  creed  as  the  other  is  heterodox,  as  dog- 
matic as  the  author  is  skeptical— let  him,  in  short,  find  his  prejudices  shocked  at 
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— still,  there  is  so  much  in  this  extraordinary  volume  to  stimulate  reflection  and 
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glad  of  having  done  so,  or,  having  begun  it,  or  even  glanced  at  almost  any  one 
of  its  pages,  to  pass  away  unread."—  London  Times. 

k'  We  have  read  Mr.  Buckle's  volumes  with  the  deepest  interest  We  owe 
him  a  profound  debt  of  gratitude.  His  irfluence  on  the  thought  of  the  present 
age  can  not  but  be  enormous,  and  if  he  gives  us  no  more  than  we  already  have 
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fanciful,  unwearied  in  research,  and  gifted  wkh  wonderful  talent  in  arranging 
and  molding  his  material,  the  author  is  as  fascinating  as  he  is  learned.  His 
erudition  is  immense— so  immense  as  not  to  be  cumbersome.  It  is  the  result 
of  a  long  and  steady  growth—a  part  of  himself."— bestow  Journal. 

IT  m> 

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in. 
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Return  to  desk  from  which  borrowed. 
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